Resonant optical cavity light emitting device

ABSTRACT

Resonant optical cavity light emitting devices are disclosed, where the device includes an opaque substrate, a first reflective layer, a first spacer region, a light emitting region, a second spacer region, and a second reflective layer. The light emitting region is configured to emit a target emission deep ultraviolet wavelength and is positioned at a separation distance from the reflector. The second reflective layer may have a metal composition comprising elemental aluminum and a thickness less than 15 nm. The device has an optical cavity comprising the first spacer region, the second spacer region and the light emitting region, where the optical cavity has a total thickness less than or equal to K·λ/n. K is a constant ranging from 0.25 to 10, λ is the target wavelength, and n is an effective refractive index of the optical cavity at the target wavelength.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/160,868, filed on Oct. 15, 2018 and entitled Resonant Optical CavityLight Emitting Device, which is hereby incorporated by reference. U.S.patent application Ser. No. 16/160,868 is related to U.S. patentapplication Ser. No. 16/115,942, filed on Aug. 29, 2018 and entitled“Resonant Optical Cavity Light Emitting Device”; U.S. patent applicationSer. No. 15/643,227, filed on Jul. 6, 2017 and entitled “ResonantOptical Cavity Light Emitting Device”; and International PatentApplication Number PCT/IB2017/050880, filed on Feb. 16, 2017 andentitled “Resonant Optical Cavity Light Emitting Device”; which claimspriority to U.S. Provisional Patent Application No. 62/298,846 filed onFeb. 23, 2016 and entitled “Resonant Optical Cavity Light EmittingDevice”; all of which are incorporated herein by reference in theirentirety.

BACKGROUND

PIN diodes are diodes with p-type and n-type regions and an intrinsicregion between. In light emitting diodes (LEDs), electrons and holesinjected from the p-type and n-type source regions recombine within theintrinsic region, generating light. The type of materials used for thep-type, i-type and n-type regions in the LED determine the wavelength oflight emitted from the device.

Simple PIN diodes typically exhibit poor light (radiation) extractionproperties which ultimately limits the useful production of light. Thisis due to the high refractive index of the materials used to make LEDsand the large number of available optical modes within the LED whichresults in a small escape cone for emitted light from the device. Lightgenerated within the device and having propagation vectors outside thiscone do not escape the device and is subjected to total internalreflection, thus being attenuated within the device and not escaping asavailable light. Typically, known LEDs exhibit approximately less than5% light extraction efficiency from a planar device, depending primarilyupon the refractive index of the material through which the lighttravels.

One method for increasing efficiency of radiation extraction in suchdevices is texturing the surface through which radiation exits thedevice. Such a textured escape surface provides a slightly larger escapecone for radiation from the device. Another method is shaping the top orbottom surface of the LED, such as in a parabolic shape, to improverefraction and reflection of light beams and thus improve lightextraction. Alternatively, researchers have also grown epitaxialsemiconductors on a textured substrate surface to improve vertical lightextraction; however, the penalty in disordered semiconductor crystalstructure has severely limited this approach. Coatings can also be usedin LEDs to affect emission of certain wavelengths of light, and thusimprove the spectral light quality produced by the device.

Different structures of light emitting diodes also present specificdesign issues for extracting light. For example, edge emitting lasersare one type of light emitting device, where light is propagated in adirection substantially parallel to the device layers and is emittedfrom the edges of the device. Numerous edge emitting LEDs and lasers arefabricated from a semiconductor wafer and are diced into individualdevices, where the cleaved, etched or cut edges become the facetsurfaces from which light is emitted. For the case of waveguide LEDs,optical reflection from the facets into the active region is suppressedby Brewster angle configuration, whereas for lasers the facets at eitherend of the planar cavity must be precisely parallel and of highreflectance. Vertical cavity surface emitting diodes (VCSEDs) and lasers(VCSELs) are another type of light emitting diode where light ispropagated substantially perpendicularly to the plane of the devicelayers and is emitted through the top or bottom surface of the device.The optical cavity can be designed with a resonance to improve emissionof a particular wavelength. An advantage of planar VCSEDs and VCSELs isthe ability to scale the output power of the device by increasing theplanar area of the device. The vertical and lateral optical confinementcan also be used to control the optical modes of the device and thusimprove the spatial emission profile and spectral quality.

SUMMARY

In some embodiments, a resonant optical cavity light emitting devicecomprises a substrate, a first reflective layer, a first spacer region,a light emitting region, a second spacer region, and a second reflectivelayer. The first reflective layer is on the substrate, the firstreflective layer having a metal composition comprising elementalaluminum and having a thickness greater than 15 nm. The first spacerregion is coupled to the first reflective layer, the first spacer regionbeing non-absorbing to the target emission deep ultraviolet wavelength(target wavelength), where at least a portion of the first spacer regioncomprises a first electrical polarity. The light emitting region is onthe first spacer region, the light emitting region being configured toemit the target wavelength. The second spacer region is on the lightemitting region, the second spacer region being non-absorbing to thetarget wavelength, where at least a portion of the second spacer regioncomprises a second electrical polarity opposite of the first electricalpolarity. The second reflective layer is coupled to the second spacerregion, the second reflective layer having a metal compositioncomprising elemental aluminum and having a thickness less than or equalto 15 nm. The first spacer region, the light emitting region and thesecond spacer region comprise AlN or AlGaN. The light emitting region ispositioned at a separation distance from the second reflective layer.The resonant optical cavity light emitting device has an optical cavitybetween the second reflective layer and the first reflective layer, theoptical cavity comprising the first spacer region, the second spacerregion and the light emitting region. The optical cavity has a totalthickness less than or equal to K·λ/n, wherein K is a constant rangingfrom 0.25 to 10, λ is the target wavelength, and n is an effectiverefractive index of the optical cavity at the target wavelength.

In some embodiments, a resonant optical cavity light emitting devicecomprises a substrate, a first reflective layer, a first spacer region,a light emitting region, a second spacer region, and a second reflectivelayer. The first reflective layer is on the substrate, the firstreflective layer having a metal composition comprising elementalaluminum and having a thickness less than or equal to 15 nm. The firstspacer region is coupled to the first reflective layer, the first spacerregion being non-absorbing to the target emission deep ultravioletwavelength (target wavelength), where at least a portion of the firstspacer region comprises a first electrical polarity. The light emittingregion is on the first spacer region, the light emitting region beingconfigured to emit the target wavelength. The second spacer region is onthe light emitting region, the second spacer region being non-absorbingto the target wavelength, where at least a portion of the second spacerregion comprises a second electrical polarity opposite of the firstelectrical polarity. The second reflective layer is coupled to thesecond spacer region, the second reflective layer having a metalcomposition comprising elemental aluminum and having a thickness greaterthan 15 nm. The first spacer region, the light emitting region and thesecond spacer region comprise AlN or AlGaN. The light emitting region ispositioned at a separation distance from the second reflective layer.The resonant optical cavity light emitting device has an optical cavitybetween the second reflective layer and the first reflective layer, theoptical cavity comprising the first spacer region, the second spacerregion and the light emitting region. The optical cavity has a totalthickness less than or equal to K·λ/n, wherein K is a constant rangingfrom 0.25 to 10, λ is the target wavelength, and n is an effectiverefractive index of the optical cavity at the target wavelength.

In some embodiments, a resonant optical cavity light emitting devicecomprises a substrate, a first reflective layer, a first spacer region,a light emitting region, a second spacer region, and a second reflectivelayer. The substrate is opaque to a target emission deep ultravioletwavelength (target wavelength). The first reflective layer is on thesubstrate, the first reflective layer being a distributed Braggreflector (DBR) superlattice that has a reflectivity of greater than 90%for the target wavelength. The first spacer region is coupled to thefirst reflective layer, the first spacer region being non-absorbing tothe target wavelength, where at least a portion of the first spacerregion comprises a first electrical polarity. The light emitting regionis on the first spacer region, the light emitting region beingconfigured to emit the target wavelength. The second spacer region is onthe light emitting region, the second spacer region being non-absorbingto the target wavelength, where at least a portion of the second spacerregion comprises a second electrical polarity opposite of the firstelectrical polarity. The second reflective layer is coupled to thesecond spacer region, the second reflective layer having a metalcomposition comprising elemental aluminum and having a thickness lessthan 15 nm. The light emitting region is positioned at a separationdistance from the second reflective layer. The resonant optical cavitylight emitting device has an optical cavity between the secondreflective layer and the first reflective layer, the optical cavitycomprising the first spacer region, the second spacer region and thelight emitting region. The optical cavity has a total thickness lessthan or equal to K·λ/n, wherein K is a constant ranging from 0.25 to 10,λ is the target wavelength, and n is an effective refractive index ofthe optical cavity at the target wavelength.

In some embodiments, a resonant optical cavity light emitting devicecomprises a substrate, a first reflective layer, a first spacer region,a light emitting region, a second spacer region, and a second reflectivelayer. The substrate is opaque to a target emission deep ultravioletwavelength (target wavelength). The first reflective layer is on thesubstrate, the first reflective layer comprising a dielectricsemiconductor. The first spacer region is coupled to the firstreflective layer, the first spacer region being non-absorbing to thetarget wavelength, where at least a portion of the first spacer regioncomprises a first electrical polarity. The light emitting region is onthe first spacer region, the light emitting region being configured toemit the target wavelength. The second spacer region is on the lightemitting region, the second spacer region being non-absorbing to thetarget wavelength, where at least a portion of the second spacer regioncomprises a second electrical polarity opposite of the first electricalpolarity. The second reflective layer is coupled to the second spacerregion, the second reflective layer having a metal compositioncomprising elemental aluminum and having a thickness less than 15 nm.The light emitting region is positioned at a separation distance fromthe second reflective layer. The resonant optical cavity light emittingdevice has an optical cavity between the second reflective layer and thefirst reflective layer, the optical cavity comprising the first spacerregion, the second spacer region and the light emitting region. Theoptical cavity has a total thickness less than or equal to K·λ/n,wherein K is a constant ranging from 0.25 to 10, λ is the targetwavelength, and n is an effective refractive index of the optical cavityat the target wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-section of a resonant optical cavity lightemitting device known in the art.

FIG. 2 is a perspective view of a deep ultraviolet resonant opticalcavity light emitting device, in accordance with some embodiments of thepresent disclosure.

FIG. 3 is a vertical cross-section of the device of FIG. 2.

FIG. 4A is a vertical cross-section of a deep UV resonant optical cavitydevice having superlattice layers, in accordance with some embodiments.

FIG. 4B is a perspective view of a deep UV resonant optical cavity lightemitting device having a superlattice reflector, in accordance with someembodiments.

FIG. 5 shows an exemplary valence band energy gap graph for the deviceof FIG. 4A.

FIG. 6 shows an exemplary luminescence graph for the device of FIG. 4A.

FIG. 7 is a flowchart of methods for producing a deep UV resonantoptical cavity light emitting device, in accordance with someembodiments.

FIG. 8A shows a schematic representation of an exemplary 3D opticaldevice and exit plane.

FIGS. 8B-8E demonstrate simulation of a deep UV resonant optical cavitylight emitting device, where the device has a square cross-section.

FIG. 9 shows an exemplary simulation output corresponding to the modelof FIGS. 8B-8E.

FIGS. 10A-10B show additional simulation outputs corresponding to themodel of FIGS. 8B-8E.

FIG. 11A shows another exemplary simulation, where the position of thelight emitting region has been altered.

FIG. 11B shows yet another exemplary simulation, where the position ofthe light emitting region has been changed to a different position fromFIG. 11A.

FIGS. 12A-12B show a 2D spatial mode pattern at the exit plane forwavelength λ=205 nm.

FIGS. 12C-12D show the 2D spatial mode pattern at the exit plane of thedevices of FIGS. 12A-12B, for wavelength λ=222 nm.

FIGS. 12E-12F show the 2D spatial mode pattern at the exit plane of thedevices of FIGS. 12A-12B, for wavelength λ=274 nm.

FIGS. 13A-13B demonstrate an embodiment of a device with an alternativeside wall angle in accordance with some embodiments.

FIGS. 14A-14B demonstrate an embodiment of another device with adifferent side wall angle from FIGS. 13A-13B, in accordance with someembodiments.

FIGS. 15A-15B demonstrate an embodiment of a rectangular device geometryin accordance with some embodiments.

FIG. 15C shows an exemplary simulated emission output for out-coupledlight at observation ports of a device.

FIG. 16A illustrates an exemplary ROCLED device.

FIG. 16B shows calculated optical modes for the device of FIG. 16A.

FIG. 16C shows optical modes for the device of FIG. 16A, for differentLER positions.

FIG. 17 is a perspective view of an exemplary device having multiplerectangular structures to create selective polarization.

FIG. 18A is a schematic of cylindrical devices for creating a singlemode output.

FIGS. 18B-18C are exemplary simulations of cylindrical devicescorresponding to FIG. 18A.

FIG. 19 is a diagram of a partially reflective mirror, in accordancewith some embodiments.

FIG. 20 is a flowchart of methods for manufacturing a deep UV resonantoptical cavity light emitting device, in accordance with someembodiments.

FIG. 21A is a perspective view of a resonant optical cavity lightemitting device with distributed Bragg reflectors.

FIG. 21B a perspective view of a resonant optical cavity light emittingdevice with metal reflectors.

FIG. 22 is a graph of a reflection spectrum of metals.

FIG. 23 is a graph of reflectance vs. wavelength for a resonant opticalcavity light emitting device with distributed Bragg reflectors, forminga fully dielectric structure.

FIG. 24 is a graph of reflectance vs. wavelength for a resonant opticalcavity light emitting device with aluminum metal high and partialreflectors forming a metal-dielectric-metal structure.

FIG. 25 is a graph of a reflectance spectrum demonstrating optimizationof an optical cavity length.

FIG. 26 shows sample values for layers of the device of FIG. 16.

FIG. 27 is a reflectance spectrum for the device of FIG. 26.

FIG. 28 shows vertical optical modes for the cavities of FIGS. 24 and25.

FIG. 29 shows an example reflector structure for an optical device ofthe present disclosure.

FIGS. 30A-30B demonstrate optimization of a titanium ohmic interlayer.

FIG. 31 shows optical cavities using dielectric mirrors andmetal-dielectric-metal.

FIG. 32 shows a reflectance spectrum for an embodiment of dielectricreflectors with bilayers of aluminum nitride and aluminum oxide.

FIG. 33 shows a reflectance spectrum for an embodiment of adielectric-semiconductor mirror.

FIG. 34 shows a reflectance spectrum for another embodiment of adielectric-semiconductor mirror.

FIG. 35 provides a view of an optical cavity device with an etchedregion in the output coupling mirror, along with a correspondingsimulation map and electric field graph.

FIGS. 36A-36B show a refractive index graph and a layer stack for anembodiment of a superlattice mirror structure.

FIGS. 37A-37B illustrate yet another embodiment of an optical cavitydevice, and a corresponding exciton absorption coefficient spectrum.

FIG. 38 is a reflectance map corresponding to the device of FIG. 37A.

FIG. 39 is an absorption map corresponding to the device of FIG. 37A.

FIG. 40 is a vertical cross-section of a deep ultraviolet resonantoptical cavity light emitting device having an opaque substrate, inaccordance with some embodiments.

FIG. 41 is a vertical cross-section of another embodiment of a deepultraviolet resonant optical cavity light emitting device having anopaque substrate.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the disclosedinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe present technology, not as a limitation of the present technology.In fact, it will be apparent to those skilled in the art thatmodifications and variations can be made in the present technologywithout departing from the scope thereof. For instance, featuresillustrated or described as part of one embodiment may be used withanother embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents.

Resonant optical cavity light emitting devices (ROCLEDs) are a type oflight emitting device which are designed to create resonance at aparticular wavelength using the principles of a Fabry-Perot resonatoralong a principal axis, to increase emission of that wavelength from thedevice. FIG. 1 shows a vertical cross-section of a general schematic ofa ROCLED 10 known in the art as a Fabry-Perot type light emittingdevice, where a light emitting region 12 (LER, also referred to as anemission or active region) is placed between two reflectors 14 and 15.The reflectors 14 and 15 are typically selected from a high reflector(reflectance) and a partial reflector, where the partial reflector isused to extract a portion of the light from within the cavity. Highquality reflectors can be made of metals, or of periodic bilayereddielectric or semiconductor materials to form distributed Braggreflectors (DBRs). For the case of metallic reflectors, the overalllength 18 of the optical cavity is the distance between the tworeflectors 14 and 15. For the case of DBR reflectors, there issubstantial spatial penetration of the optical field into the DBR whichstrongly depends upon the refractive index difference between thequarter-wave thickness materials used to form the DBR. This DBRpenetration distance is particularly disadvantageous for application toultraviolet application, as it may become of the same length scale asthe desired optical cavity length, thereby rendering wavelength scalecavity formation challenging. Furthermore, for application toultraviolet wavelengths the selection of materials suitable for DBRformation are severely limited and the desire for large refractive indexdifference between quarter wave layers further places limits onavailable materials.

In the description of FIG. 1, the device will be considered as aresonant optical cavity formed with negligible penetration depthreflectors. A spacer 16 is positioned between the upper reflector 14 andthe LER 12, and another spacer 17 is between the lower reflector 15 andthe LER 12. The resonant wavelength of the LED is determined by thedimensions of the optical cavity, such as the overall length 18 of theoptical cavity and the lengths of the spacers 16 and 17. Note that inthis disclosure, quantities labeled as a vertical length “L” in thefigures may also be referred to as a thickness, such as a thickness of aparticular region or layer. When electrical current is applied to thedevice 10, electrons and holes created in the n-type and p-type regions(spacers 16 and 17) are transported spatially into the i-region (LER 12)by drift and diffusion processes where they can recombine, generatingspontaneous emission of light within the active region (LER 12).Furthermore, if excitation of simultaneously large electron and holecarrier densities in the LER can be achieved within upper excited statesand with sufficiently long lifetime before relaxing to a lower energystate, then stimulated emission is possible. Such an electronic upperstate configuration can be designed by appropriate design ofsemiconductor quantum mechanical states, such as quantum well andsuperlattice heterostructures. This excited LER active region can thenbe subject to stimulated emission process by virtue of the photonrecycling within the cavity due to the mirrors (reflectors). Apropagating photon within the cavity of energy equal to the transitionenergy of the electron-hole pair is stimulated to relax from the excitedcarrier states into a lower energy or ground state. Such a resonantoptical cavity device can therefore become a semiconductor laser.

Although resonant optical cavity LEDs are known in the art, they havenot been used for deep ultraviolet (deep UV, or DUV) wavelengths whichoperate in the 200-280 nm range. DUV wavelengths present new challengesin terms of the materials and design criteria that must be overcome,which are not a straightforward extension of existing designs. In thepresent disclosure, resonant cavity light emitting devices for deepultraviolet wavelengths with improved efficiency are presented thatovercome issues unique to this wavelength range.

FIG. 2 is an exemplary embodiment of a deep ultraviolet resonant opticalcavity light emitting device 100 according to the present disclosure.FIG. 2 is a perspective view showing a simplified diagram, for clarity.Device 100 has: a transparent substrate 110 which may be, for example,glass or sapphire or calcium fluoride; a first substantially transparentspacer 120 on the substrate 110; a radiation emission region 130 whichis an active or intrinsic region where electrons and holesadvantageously combine to emit UV radiation typically at about 280 nm orless; a second substantially transparent spacer 140; and a reflector 150above the second spacer 140. Device 100 has a cylindrical geometry inthis embodiment. The resonant optical cavity has a length L_(CAV)between the reflector 150 and the first surface 112 of the transparentsubstrate 110. The radiation or light emission region or LER 130 has alength L_(EM), first spacer 120 has a length L_(SEP1), and second spacer140 has a length L_(SEP2). L_(CAV) is typically chosen to be theequivalent to an optical thickness which can be selected to be amultiple of a half-wavelength of the target wavelength to be produced.In general, L_(CAV)=m·λ/n(λ), where m>0, λ is the desired operatingwavelength and n(X) is the effective refractive index of the cavitymaterial at λ.

A large variety of optical cavity devices are possible depending uponthe reflector properties and the effective optical thickness withrespect to the operating wavelength. For example, sub-wavelength andwavelength scale optical cavities are possible using low opticalpenetration depth reflectors such as metallic mirrors. Cavity lengthsgreater than the operating wavelength and multiples thereof are alsopossible. The effective optical thickness of the cavity determines thelongitudinal mode density and frequency spacing of the modes within thedevice. Reducing the total number of non-propagating optical modes forlight generated within the structure and increasing the overlap ofpropagating optical modes with the LER region may be beneficial forimproving the light extraction efficiency.

The reflector 150 is a high UV reflectance metal, chosen preferably tobe aluminum (Al) for the desired emitted radiation in the deep UV range.The aluminum metal reflector 150 of the present disclosure provides avery small penetration depth for the deep UV radiation emitted by theradiation emission region 130. For integration into group-III metalnitride epitaxy—the AlN-based semiconductors—at least a portion of theAl-metal reflector 150 can be deposited in-situ within the epitaxialdeposition system by virtue of the Al metal being one of the constituentdeposition sources. In conjunction with in-situ film thicknessmonitoring, this in-situ reflector process can be used to precisely forman optical cavity device as described herein.

The LER 130 is configured such that UV radiation with a wavelength ofless than 280 nm is emitted. Possible materials for light emittingregion 130 include, for example, group III-N materials such as AlN, GaN,and AlGaN, and superlattice configurations of these materials. In someembodiments, the LER 130 may include one or multiple quantum wells. Inyet another embodiment, the LER 130 may be a bulk-like AlGaN alloy thatis subject to external mechanical stressors to tune the desiredoptoelectronic properties. The first spacer 120 may comprise at least aportion of an n-type material with the second spacer 140 being at leasta portion of p-type, or vice versa. The spacers 120 and 140 aresubstantially transparent—that is, non-absorbing or opticallytransparent—to the desired wavelength (also referred to as a targetwavelength in this disclosure) emitted by the device. For example, thefirst and second spacer regions 120 and 140 may be at least 80%transparent to the target wavelength produced by device 100, such asgreater than 90% transparent. The refractive index of the substrate 110is preferably less than the refractive index of the spacers 120 and 140.The refractive index difference between the first spacer region 120 andthe transparent substrate 110 forms a partially reflecting interfacethat can be further used to tune the optical cavity.

In other embodiments, substrate 110 may be opaque to the targetwavelength. The opaque substrate can then have an optical port etchedthrough the substrate and terminating at least at the first spacerregion. An optional coating can further be deposited on the exposedportion of the first spacer to form a second reflector. For example, aBosch etch process can be used to etch an optical via beneath the activeregion of the optical device. A partially transmitting metal reflectorcan then be deposited to form the optical cavity. Alternatively, one ormore dielectric transparent coatings can be applied to the exposedportion of the first spacer, such as, a high-low index DBR ormetalo-dielectric DBR, as disclosed herein.

FIG. 3 shows a vertical cross-sectional view of the deep UV LED device100, demonstrating the propagation of light. Light generated in thelight emitting region 130 will propagate outward, such as in a downwarddirection as indicated by ray 180 a and in an upward direction asindicated by ray 180 b. For ray 180 a, some light will be reflected atthe interface between substrate 110 and first spacer 120, due to thedifference in refractive indices of these two materials. Ray 180 a willthen reflect off reflector 150, and then ultimately will be emittedthrough transparent substrate 110 (or through an optical port in thesubstrate if substrate 110 is opaque). For ray 180 b, light will bereflected off reflector 150 and then exit through substrate 110. Notethat FIG. 3 is a simplified diagram, in that the light rays 180 a and180 b will have additional propagation paths due to varying amounts oftransmission and reflection at the interfaces between layers of thedevice 100. Furthermore, three dimensional devices will further havelateral features that participate in optical reflection and absorption.

The reflector 150 may consist of a planar Bragg grating or be apatterned metallic reflector. The material for reflector 150 must have alow absorption (i.e. high reflectance >80% at the target operatingwavelength) and small penetration depth at the wavelength of the deviceif sub-wavelength optical cavities are desired. Typically, thepenetration distance into an Al metal reflector is <1 Å. For deepultraviolet wavelengths, metals other than aluminum tend to absorb toomuch of the radiation at the wavelength emitted by the device. Thus,aluminum is the material of choice for the reflector 150 of deep UVdevice 100. This is demonstrated in the graph of FIG. 22, where thereflectivity of aluminum (horizontal line with open circles) is greaterthan 0.9, compared to the other listed materials which havereflectivities less than 0.7. An aluminum reflector 150 may have athickness large enough, such as greater than 15 nm thick, to reflect theemitted light out through the substrate 110. However, utilization ofaluminum as a mirror material with a III-N device is challenging sincethe epitaxial processes that are needed for fabricating LEDs typicallyrequires that p-type layers are grown last. Although aluminum is theonly metal with high reflectance in the DUV range, it is a poor ohmicmetal to serve as an electrical contact to p-type III-N. P-metalcontacts are ideally high work function metals such as osmium, nickeland palladium for ohmic contact to p-type III-N. However, Os, Ni and Pdare high UV-absorbing materials. Therefore, using these materials aselectrical contacts for the present deep UV device would result inplacing a UV-absorbing material between the active layers of the deviceand the reflective mirror Al. Additionally, thin p-type GaN layers aregenerally necessary for metal polar “P-up” structures (devices with thep-type layer on upper end of the device, opposite the substrate at thebottom of the device), enabling a two-dimensional hole gas to exist atthe pGaN/AlGaN or pGaN/pSL (superlattice “SL”) interface. This helps pinthe valence band structure for PIN diode operation. pGaN is also highlyUV-absorbing, which also is not ideal for a UV device. In the presentdisclosure, these issues may be optimized for a given UV operatingwavelength.

FIG. 4A is a detailed cross-section of an exemplary deep UV resonantoptical cavity light emitting device 200 in which electrical contactsare shown and the layers comprise superlattices. Device 200 includessubstrate 210, n-type first spacer region 220, light emitting region 230having thickness L_(EM), p-type second spacer region 240 having lengthL_(SEP1), and optical reflector 250. Thus, device 200 is a P-up device,since the p-type layers are furthest from the substrate 210. Device 200also includes an optional buffer layer 260 between the substrate 210 andfirst spacer 220. An optical cavity having a total thickness L_(CAV)between the upper surface of substrate 210 and the lower surface ofreflector 250 is comprised of first spacer region 220, LER 230, andsecond spacer region 240. L_(CAV) also includes buffer layer 260 in thisembodiment. The buffer layer 260, n-type spacer 220 and p-type spacer240 form the majority of the optical cavity. First spacer 220, lightemitting region 230 and second spacer 240 are superlattice (SL)structures in this embodiment. First spacer 220 can include at least aportion of n-type SL and a portion of i-type SL, LER 230 is an i-typeSL, and second spacer 240 can include both intrinsic and p-dopedmaterials from superlattice layers 232 and 242. The selection of the SLstructures comprising the n-type, i-type and p-type regions is toprovide the desired electronic and optical functions necessary for thedevice operation (as disclosed in U.S. Patent Publication No.2016/0149075 entitled “Optoelectronic Device”). Alternatively, thesemiconductors can be formed using bulk-like semiconductors with the LERcomprising one or more quantum well regions for wavelength tuning.

The material for reflector 250 is aluminum to provide the leastabsorption of the deep UV wavelengths and highest reflectance. However,as discussed above, Al does not provide optimum electrical contact withp-type materials. To overcome this issue, the electrical contact for thep-type layer of device 200 is configured as a compound metallic contactin which ohmic contact 270 is in contact with p-type second spacer 240,on the same layer and at opposite ends of the reflector 250. The ohmiccontact 270 and reflective contact 250 are spatially separate across thep-type spacer 240, and this opens the design for near diffractive freeaxicon emitters. Various configurations are possible but must optimizethe series resistance. An optional p-type GaN contact layer 275 may beplaced between second spacer 240 and ohmic contact 270. To form theelectrical contact area of the device 200, the Al metal reflector 250may be deposited in-situ on top of the epilayer stack. Subtractivepatterning may be used to form the remainder of the device geometry. TheAl reflective portion 250 in the DUV device 200 may be created using aCMOS-style processing method free from a flip-chip process that iscompatible with interlayer dielectric (SiO₂) and metal (Al) interconnectprocess.

In other embodiments, the device 200 may be configured as a p-type downepilayer stack where first spacer 220 is p-type and second spacer 240 isn-type. Al-metal for reflector 250 in such a P-down configuration is ontop of n-type region. For a P-down device, pure Al or Ti/TiN/Al can beused as an n-type contact on the backside (above second spacer 240).Selecting the Ti and or TiN layers to be sufficiently non-absorbingassists in coupling the optical cavity to the Al reflector. The opticalcavity or reflective backside LED for a P-down device can therefore usea simple Al-metal contact.

The reflector 250 may also be a superlattice structure, as shown in FIG.4B. Deep UV reflectors using conventional AlN, AlGaN and GaN quarterwavelength (λ/4) layers for distributed Bragg reflectors (DBRs) arechallenging for the target operating wavelength being less than 280 nm.This is because of the strong optical absorption of the bulk materials(particularly GaN), and an extremely large number of periods beingrequired to achieve high reflectance (since GaN, AlGaN and AlN materialsoffer only a small change in refractive index for implementing DBR).Indium aluminum nitride (InAlN) is also possible but suffers the samelimitations as well as being challenging to fabricate. Also, thesematerials are difficult to grow with low defect density and lowmicroscopic and macroscopic film stress—as crystalline materials whichare further challenging to electrically dope into n-type or p-typeconductivity. Note also that a mole fraction of x>0.7 in Al_(x)Ga_(1-x)Nis required to attain sufficient transparency for operating wavelengthless than 280 nm, which further challenges the utility of DBRs for thepresent application using native materials. In embodiments of thepresent disclosure, a new class of optical structure based on ultrathinmetallic layers forming an Al/AlN superlattice may be used to form oneof the quarter wave layers comprising a DBR. This is advantageous sinceelemental Al-metal has an extremely low refractive index, and ultrathin(e.g., <2 nm) Al films can be tailored to have an absorption coefficientthat is acceptable. This is by virtue of metallic quantum confinement ofthe dielectric-metal-dielectric AlN/Al/AlN structure which can betailored for advantageous optical transparency at the desired operatingwavelength. The resulting film is a metallic/dielectric superlattice,where the DBR is based on a plurality of λ/4 slabs comprising AlN andSL[Al/AlN]. Such a structure is defined herein as a metalo-dielectricreflector. The number of periods of the SL[Al/AlN] is determined by theeffective refractive index n_(EFF) of the superlattice thickness neededto create the λ/(4 n_(EFF)). FIGS. 21A-21B show a comparison ofreflector types, where FIG. 21A shows a ROCLED with DBR reflectors andFIG. 21B shows a ROCLED with metal reflectors. As can be seen, thealuminum reflectors of FIG. 21B enable an extremely thin structure forthe ROCLED.

Yet a further embodiment for implementing a high quality DBR in theultraviolet wavelength range uses quarter wavelength layers comprisingAlN and magnesium fluoride (MgF₂). The DBR and metalo-dielectricreflector methods are alternatives to using a pure Al metal reflector.An exemplary reflectance spectrum for a resonant optical cavity devicewith DBR reflectors using AlN and MgF₂ quarter wave layers is shown inFIG. 23.

FIGS. 21A and 21B represent exemplary vertical optical devices, FIG. 21Aforming a fully dielectric optical cavity with reflectors formed usingtwo DBR structures, and FIG. 21B being a metal-dielectric-metal opticalcavity formed using high reflectance and semitransparent metallicreflectors. The device in FIG. 21B attains very small optical cavitythicknesses less than the optical emission wavelength. Experimentalresults corresponding to an example fully dielectric optical cavitystructure are shown in the graph of reflectance vs. wavelength in FIG.23, using a high reflectance 20 period AlN/MgF₂ DBR, a half-wavelengththick AlN cavity and partially reflecting output coupler comprising a5.5 period AlN/MgF₂ DBR. The normal incidence reflectance from the 5.5period DBR is shown as function of wavelength. The single optical modeat λ=250 nm is clearly evident and the out of band rejection away fromλ=250 nm exhibits extremely high reflection. A high reflectanceindicates that optical modes at the out of band wavelengths areforbidden within the device and thus represent non-propagating opticalmodes vertically within the device. Conversely, the λ=250 nm opticalmode is allowed and is the optical cavity mode desired. The DBRs aredesigned to have quarter wavelength thickness at λ=250 nm comprising thebilayer pairs. Note the additional phase matching half layer in thelower reflectance DBR to ensure optical phases are constructive.Conversely, the reflectance spectrum graph for a metal-dielectric-metalstructure in FIG. 24 shows the normal incidence reflectance at thepartially reflecting metal layer comprising 15 nm of Al. Variousstructures are shown with the AlN optical cavity having thicknessranging from ½ wavelength to 7×½ wavelength thickness. Tuning of theoptical cavity mode is clearly evident ranging from 240 nm through to290 nm. The metal-dielectric-metal optical cavity is therefore aversatile optical device being able to provide the required opticalcavity tuning range and high out of band rejection.

FIGS. 21A-21B represent various embodiments of low (i.e., partial)reflectors between the substrate (e.g., substrate 210 of FIG. 4A) andfirst spacer (e.g., first spacer region 220 of FIG. 4A). FIG. 21Arepresents a superlattice (SL) DBR having two types of regions (SLlayers), each having quarter-wave thicknesses as explained in relationto FIG. 4B. One region of the DBR of FIG. 21A may be ultrathin Al/AlN SLlayers having thicknesses of less than 20 nm, such as less than or equalto 15 nm (i.e., over an order of magnitude less than deep UV quarterwavelengths), and the other region may be an AlN region of deep UVquarter wavelength thickness. The number of repetitions of these tworegions can be a low number of periods which is chosen to achieve thedesired low reflectance, such as <10% reflectance. Additional examplescorresponding to the use of partial reflector DBRs between the substrateand first spacer layer are shown in FIGS. 23, 31 (device structure 3105;note the substrate is not shown), 33, 34 and 35. FIG. 21B represents apartial reflector made of an ultrathin Al layer having thickness lessthan 20 nm, such as less than or equal to 15 nm (again, much less thandeep UV quarter wavelengths) to achieve low reflectance of, for example,<10%. Additional examples corresponding to the use of partial reflectorsusing thin film Al-metal are shown in FIGS. 18A, 19, 24, 25 and 31(device 3150 of FIG. 31; note the substrate is not shown in allexamples).

In other embodiments, the device of FIG. 21A or 21B may omit thepartially reflecting (low reflector) output coupler; that is, no DBR orthin Al layer is present between the substrate and first spacer. In sucha device, the substrate can be directly coupled to the first spacerregion and be optically transparent to a target emission deepultraviolet wavelength. The high reflector on the second spacer ishighly reflective of the target wavelength, such as having areflectivity of greater than 90% for the target wavelength. The highreflector can be a bulk-like aluminum metal layer, or a DBR comprisingone or more of Al, MgF₂ or AlN as described above. Light emitted fromthe LER will be output through the transparent substrate. The device maybe either p-up or p-down.

In some embodiments, partial reflection between the substrate and firstspacer region can be achieved without the presence of a physical partialreflector layer. For example, the partial reflector layer may be omittedin embodiments in which a particular selection of III-N materials isselected to have a sufficient difference in refractive index to thesubstrate material. The difference in refractive index between thesubstrate material and other layers can provide the partial reflectancesuch that the need for a specific partial reflector layer is eliminated.For instance, the first and second spacer layers, active layers, p-typelayers and n-type layers may all be implemented to have a sufficientdifference in refractive index from the substrate material. Removing therequirement for an additional partial reflector layer between the firstspacer region and the substrate greatly simplifies the manufacturabilityof the device. A typical substrate material is single crystal sapphire,whereby the difference in the ordinary refractive indices Δn between afirst spacer layer comprising AlN (n=2.396 at 250 nm) and the substrateof Al₂O₃ (n=1.845 at 250 nm) is Δn=(2.396−1.845)=0.551. This is asufficiently large discontinuity in the optical property of the layersat the interface Δn (difference in refractive index between thesubstrate and first spacer region) to produce a substantial refectionfor light rays directed from within the AlN layer toward theAlN/sapphire interface. In embodiments without a partial reflectorbetween the substrate and first spacer, it is this AlN/sapphireinterface that serves as the output coupler or partial reflector of thedevice. This property has been overlooked in the prior art as a methodfor producing microcavity effects. The effect described above may alsobe further enhanced by use of AlGaN ternary or AlN/GaN superlatticematerials comprising the first spacer layer and a sapphire substrate.Indeed, an Al_(x)Ga_(1-x)N alloy (for example x=0.7) or AlN/GaNsuperlattice material (with average Al alloy content of 70%) comprisingthe first spacer region will also have sufficient Δn even if a bulksingle crystal AlN substrate is used. The partial reflectance asdescribed by the abrupt discontinuity in refractive index at the firstspacer layer and substrate is further exploited when a high reflector isplaced as described on the opposite portion of the device from thesubstrate thereby forming the optical cavity. The high reflector willexacerbate the effect of the partial reflector interface by virtue ofmultiple reflections of light rays within the volume of the devicecavity.

FIG. 5 shows a graph of the lowest energy zone center conduction andvalence bands of the direct energy gap materials GaN and AlN forming thesuperlattice layers of device 200 of FIG. 4A, in certain embodiments.The superlattice of GaN/AlN ultrathin layers forms a pseudo-alloy withthe lowest energy quantized energy states forming the majority of theelectronic and optical properties. The graph of FIG. 5 illustrates aP-up device, where the n-type layers are grown first and the p-typelayers are grown last. An optional pGaN contact layer is also shown(growth layer starting at 140 nm). In the graph of FIG. 5, the p-typeand n-type superlattice (SL) layers of FIG. 4A are designed to besubstantially transparent to the desired emission wavelength λ_(EM). TheNID (non-intentionally doped) i-type SL region is designed for emissionat the desired emission wavelength λ_(EM). The vast majority of theprobability for electron and heavy-hole wavefunction overlap isspatially confined within the i-SL region. In the present disclosure,the i-SL region can therefore be positioned vertically within the PINstructure and also placed optimally between the Al-metal reflector andthe low refractive index substrate (e.g., sapphire). For application toultraviolet operation using polar and non-polar group-III metal nitridesemiconductors, the utility of superlattice structures comprisingdesigned AlN/GaN layers enables vertically emissive devices and theability to bandgap engineer the device with only two materialcompositions. However, other methods such as using conventionalbulk-like Al_(x)Ga_(1-x)N ternary alloys are also possible for thedoped, spacer and LER regions. Yet further embodiments include the useof superlattices comprising Al_(y)Ga_(1-y)N/Al_(x)Ga_(1-x)N andtrilayered AN/Al_(x)Ga_(1-x)N/GaN, where x≠y.

FIG. 6 shows an exemplary transverse electric (TE) luminescence spectrumfor device 200, where the LED stack has been designed such that thei-type SL region has a narrow emission spectrum clear of the absorptionedge of the doped cladding layers. The lowest energy quantized states ofthe SL structure generate an emission spectrum according to the spatialoverlap of the electron and hole quantized wavefunctions. The AlN/GaNsuperlattice optical emission is dominated by the lowest energyquantized electron wavefunction spatial overlap with the lowest energyquantized heavy-hole wavefunctions. There are three important valencebands comprising the superlattice, namely, the heavy-hole (HH),light-hole (LH) and spin-split off (SO) band. Using a GaN/AlNsuperlattice ensures the vertical emission is dominated by theadvantageous TE emission due to recombination between the lowest energyelectron states with those from the HH band. The optical modes of thedevice dictate which spatial and wavelength modes are coupled out of thedevice. That is, the i-SL emission energy generating the desiredemission spectrum must therefore be matched to propagating optical modesout of the device. That is, the optical modes of the device aredetermined by the optical constants of the materials comprising thedevice and the specific 1D and 3D structure. That is, forming a devicethat is comparable in length scale to the emission wavelength requirescareful attention to the way light interacts at interfaces anddiscontinuities. This method offers the highest extraction efficiencypossible. In embodiments of the present disclosure, the optical geometryof the device stack can be simulated using a three-dimensionalelectromagnetic spatial and temporal simulator, such asthree-dimensional finite-difference time domain (3D FDTD) methods, tomaximize the emission of light from the device at the desired emissionwavelength. Another method that can be used to perform the opticaldesign is the one-dimensional transfer matrix method (1D TMM), whichutilizes the reciprocity between: (i) absorption within spatial regionsof the device when illuminated through the substrate; and (ii) emissionof light from a spatial position within the device and detected externalto the substrate. While the 3D FDTD method provides 3D details of thephysical device fabricated, namely, sidewall slopes and surfaceroughness and the like, the 1D TMM provides a fast method of determiningthe longitudinal modes of the target structure and advantageouspositioning of the LER with respect to the allowed optical longitudinalmodes.

FIG. 7 provides a flowchart 700 of methods for producing a resonantoptical cavity light emitting device that meets a predeterminedcriterion at a target emission deep ultraviolet wavelength. The targetlight emission output at a selected wavelength frequency is dependentupon the positioning of the LER provided by the two spacers within thecavity. The process illustrated in FIG. 7 shows how the structure of thedevice may be arrived at. First, in step 710, a target emission deepultraviolet wavelength of the ROCLED is selected. The wavelength is lessthan or equal to 300 nm, such as between 190-300 nm, or 200-280 nm, or250-280 nm. The LED has a light emitting region between a substrate anda reflector, where a first spacer region is between the light emittingregion and the substrate, and a second spacer region is between thelight emitting region and the reflector. The reflector for the deep UVdevice comprises a metal composition comprising elemental aluminum. Atleast one of the light emitting region, the first spacer region, and thesecond spacer region may comprise a superlattice. In some embodiments,the material for the substrate may be selected as part of the processfor producing the ROCLED, such as by choosing an optically transparentsubstrate with optical properties suitable for the emission wavelengthto be generated.

Next in step 720, the desired dimensions are selected, including aseparation distance between the light emitting region and the reflector.The particular location of the LER within the cavity length L_(CAV) maybe expressed as, for example, L_(SEP1) and/or L_(SEP2). In someembodiments, step 720 may selecting a thickness of the first spacerregion and selecting a thickness of the second spacer region to matchthe target wavelength to at least one allowed optical mode formed by theoptical cavity and the reflector. In other embodiments, the simulatormay be used to calculate a possible separation distance between thelight emitting region and the reflector.

In step 730 the magnitude of the radiation emission at an exit planerelative to the substrate is determined using a three-dimensionalelectromagnetic spatial and temporal simulator, such as a 3D FDTD. Thepredetermined criterion is selected from i) an optical extractionefficiency at an exit plane with respect to the light generated from thelight emission region at the target wavelength, ii) a target spatialprofile of light emitted at the exit plane at the target wavelength, andiii) light extracted from the device being emitted substantiallyperpendicular to the substrate. The quantized states generate anemission spectrum. The optical modes of the device dictate which spatialand wavelength modes are coupled out of the device. The i-SL emissionenergy must be matched to propagating optical modes out of the device.In some embodiments, the optical geometry of the device stack can besimulated using at least one of a 3D FDTD and a 1D TMM. For example, the1D TMM can be used to directly simulate the electromagnetic fieldpropagation within the entire structure including the wavelengthdependent refractive indices and absorption coefficients for all theconstituent materials. Using the optical reciprocity between absorptionand emission, the 1D optical structure can be illuminated through thetransparent substrate with a range of wavelengths. By further using asufficiently thin absorption test layer that is scanned as a function ofposition within the optical cavity, the absorption coefficient withinthe test layer can be calculated to reveal the 1D longitudinal opticalmodes. Positioning the LER at an allowed optical mode (namely, highabsorption) is advantageous for optimizing the light absorption at theLER. Conversely, light emitted at this selected LER position alsoenables light to be out coupled through the substrate with the highestextraction efficiency.

For example, FIGS. 16A-16C can be used to describe the method. FIG. 16Ashows how the 1D (longitudinal) optical modes can be discovered for thecase of an exemplary AlN cavity with a sapphire substrate and Alreflector. In practice P-up structures require the use of interposingp-GaN and ohmic metal (such as, Os, Pt or Ni) layers between the AlNcavity and the Al reflector. Optimizing the absorptive properties of thep-GaN and ohmic metal layer thickness for minimizing disadvantageousabruption at the desired ultraviolet operating wavelength is possible byminimizing the respective thicknesses. FIG. 16A is an example opticalcavity device 1600 comprising sapphire substrate 1601, and spacer 1602with length L_(cav1) (indicated as distance Δ 1630) and made of AlN orhigh percentage Al superlattice. Optical cavity device 1600 alsoincludes LER 1603 (also referred to as electron-hole recombinationregion, EHR), spacer 1604 with length L_(cav2) and made of AlN or highpercentage Al superlattice, p-region 1605 of pGaN (UV absorbing), highwork function metal 1606 (UV absorbing), and aluminum UV high reflector1607. The substrate 1601 has lower refractive index than the dielectricmaterials. LER (i.e., EHR) region 1603 is advantageously positionedwithin the dielectric cavity at distance A 1630. This example AlN-baseddevice that includes a p-type contact stack (1605 and 1606) positionedbetween the high reflector 1607 and spacer 1604 is optically absorptivewith respect to the operating wavelength of the device. The opticallyabsorptive layers 1605 and 1606 form the ohmic contact stack to thep-type gallium nitride (GaN) layer 1605. The optical emission/absorptionoperation of the device can be simulated by optimizing theemission/absorption within the EHR for optical wavelengths of interest.

FIG. 16B shows the calculated optical modes as a function of inputwavelength to the device through the substrate and represented asabsorption resonances 1640 of test layer (i.e., LER or EHR region) 1603as a function of vertical position 1630 within the stack of opticalcavity 1600. Clearly, high absorption optical modes can be determineduniquely for a given structure. FIG. 16C exemplifies the requirement forpositioning the LER region having a specific optical emission spectrum1650 within the cavity. For example, FIG. 16C shows an LER spectrum 1650peaked at λ=265 nm, which overlaps an allowed optical mode spectrum 1655of the cavity 1600 if and only if the LER is positioned at a distance ofΔ=735 nm from the substrate (e.g., sapphire) surface. In contrast, ifthe LER is positioned at Δ=706 nm with optical mode spectrum 1660 whichrepresents an interference null within the cavity, the LER spectrum 1650cannot couple to an allowed optical propagating mode and thus light isnot emitted from the sapphire due to an LER positioned at Δ=706 nm.

Alternatively, in step 730 of FIG. 7, a more complex and computerintensive 3D FDTD method can be used to simulate the germane lightpropagation as a function of time at a given LER position within theoptical cavity. The propagated electromagnetic radiation can be timesequentially stepped until the light propagates throughout the structureand sampled at a desired exit plane. In one embodiment of a method forsimulating the 3D ROCLED is the construction of a discrete 2D ensembleof randomly polarized electric dipole emitters forming the LER region.Furthermore, these electric dipoles are simultaneously excited by anultrashort optical pulse that is specifically constructed to support asufficiently wide optical spectrum. For example, a Gaussian femtosecondpulse can be constructed to support a continuous range of wavelengthsspanning from 170 nm to about 2 microns. The optical pulse is propagatedin discrete time steps (using the leap frog method of the known LeeAlgorithm) until the entire structure is illuminated by the ultrashortpulse and for a sufficiently long time so that the propagatedelectromagnetic waves pass through the desired exit plane. The discretetime sequence of the electromagnetic field vectors as sampled at theexit plane can then be applied to a Fourier transform to reveal thefrequency spectrum. That is, the impulse response transfer spectrum ofthe device as sampled at the exit plane is established for a givenoptical cavity and LER position configuration. This method also enablesrevealing the spatial mode pattern for a particular wavelength at theexit plane and is beneficial for understanding the impact of the lateralstructure on the resulting optical spatial profiles.

The LER position within the optical cavity has a cavity pulling effecton the luminescence spectrum. That is, if the emission spectrum of theLER overlaps a single and narrower optical mode then the resulting lightextracted through the substrate will be commensurate with the allowedoptical mode linewidth. Changing the LER position results in differentabsorption vs. wavelength responses. If injected electrons and holesrecombine spatially within an allowed node in the cavity structure, thenthe light can be generated. Otherwise, spontaneous emission isinhibited. Therefore, the LER needs to be placed at a predeterminedposition within the epi-stack forming the cavity. In some embodiments,periodic electron-hole recombination sections of the superlattices canbe grown in the epi-stack to match the cavity modes. The light emittingregion may include a plurality of light emission regions separated byfurther spacer layers, thereby forming a periodic emission region thatoverlaps allowed spatial modes of the optical cavity.

In step 740, if the determined emission output of step 730 does not meetthe predetermined criterion, such as by not meeting a specifiedmagnitude within a certain tolerance, then a different position for theLER is selected. The steps of selecting a separation distance in step720, determining the emission output in step 730, and determining if theemission output meets the predetermined criterion are then repeatedusing alternative values for the separation distance. The process isrepeated until an optimized value of the separation distance, whichshall be defined as a final separation distance in this disclosure,results in the predetermined criterion meeting a desired efficiency.Thus, after the emission output is acceptable at step 740, the lightemitting region is placed at the final separation distance from thereflector in step 750.

FIGS. 8-12 provide exemplary illustrations of the simulation process fordetermining the position of the light emitting region within the deep UVdevice. FIG. 8A is an isometric view of an exemplary resonant opticalcavity light emitting device 800 and a two-dimensional exit plane 801where a desired output or efficiency is to be evaluated. FIGS. 8B and 8Care vertical cross-sections taken orthogonally to each other (across thex- and z-dimensions), showing a device 805 similar to device 800, wherethe substrate 810 is facing upward. First spacer 820 is between thesubstrate 810 and LER 830, and second spacer 840 is between the LER 830and reflector 850. FIG. 8D is a top view, showing that the device 800has a square cross-section. The side walls of the device 800 are angled,where the layers have a decreasing area toward the reflector 850 in thisembodiment. For example, a cross-section of the first spacer 820 isgreater than a cross-section of the second spacer 840. FIG. 8D shows theoutlines of the various layers: substrate 810 (SUB boundary), firstspacer 820 (Base MESA), LER 830 (Active-SL volume), and reflector 850(AL-metal). FIGS. 8B-8D also show three locations above substrate810—port 1, port 2, and port 3—defining a two-dimensional exit plane atwhich a desired output, such as a magnitude of radiation emission, willbe evaluated. FIG. 8E shows a source volume polarization map forE_(Z)-dipoles in the active layer 830 demonstrating the 2D discreteensemble of emitters each of which is excited by the Gaussian ultrashortpulse used to form the impulse response transfer spectrum. Thedimensions of the LED are input into the FDTD simulation, including theposition L_(SEP) of the light emitting region 830 relative to thereflector 850. L_(SEP) may be input as an absolute distance, such as inmicrometers, or as a proportion, such as a fraction of the length of thecavity.

FIG. 9 shows an exemplary output of the modeling at an instant of time,t=6.7 fs in this sample result. Since the light from LER 830 undergoesmultiple reflections that interact with each other as they propagate,the spatial E_(Z) fields are time-dependent. Furthermore, thethree-dimensional geometry of the device affects the emission of lightfrom the device. The two-dimensional exit plane defined by ports 1, 2and 3 of FIGS. 8B-8D is indicated by the red dashed line. The exit planeis where the magnitude of radiation emission will be measured. In FIG.9, some radiation fields at this time point exist within the devicewhile some have been emitted from the device toward the exit plane.

FIG. 10A shows the simulation results at a later time t=12.7 fs, atwhich point the emission has reached the exit plane. FIG. 10B shows atop view of emission at the exit plane, at approximately the same timepoint as FIG. 10A. FIG. 10B shows that a non-uniform distribution oflight in the x-z plane occurs. This distribution changes over time.Thus, the prediction of the device efficiency is a complexthree-dimensional problem that is uniquely addressed in this disclosureby using finite-difference time-domain modeling to optimize the positionof the active region within the LED.

FIGS. 11A-11B show further iterations of the simulation, where thevalues of L_(SEP) are varied and the resulting magnitudes of radiationemission are determined. In FIG. 11A L_(SEP) is less than the value usedin FIGS. 9-10, and in FIG. 11B L_(SEP) is greater. The graphs in FIGS.11A and 11B show the calculated magnitude of radiation emission at theports of the exit plane, for various wavelengths produced by the device.In the graph of FIG. 11A, the radiation magnitude is best for awavelength of approximately 300 nm, while in FIG. 11B the emissionstrength varies greatly with wavelength. From these graphs, through thesimulation methods of the present disclosure, an optimal position of thelight emitting region for the device can be determined, to provide adesired efficiency at a target wavelength.

FIG. 12A-12E show sample calculated 2D spatial optical mode profiles atthe exit plane for three specific wavelengths λ=205, 222 and 275 nm withthe effect of the position of the LER within the cavity, whereΔ=fraction/L_(CAV) represents the fractional position relative to thetotal cavity length L_(CAV). Selected example positions are fraction=⅙representing an LER close to the reflector, and fraction=⅚ representinga position close to the substrate. The 2D mode pattern is shown to haveuniform spatial power over a majority of the active device forfraction=⅙ for all three wavelengths, whereas the LER positionedfurthest from the Al mirror (fraction=⅚) displays a multimode spatialpattern that is disadvantageous for external power coupling.

Using the methods of designing a ROCLED as described herein, a resonantoptical cavity light emitting device is produced. In embodiments of thepresent disclosure, the ROCLED includes a substrate, a first spacerregion coupled to the substrate, a light emitting region on the firstspacer, a second spacer region on the light emitting region, and areflector coupled to the second spacer region. The first spacer regionis non-absorbing to a target emission deep ultraviolet wavelength, whereat least a portion of the first spacer region comprises a firstelectrical polarity. The light emitting region is configured to generatelight at the target wavelength, and is positioned at a separationdistance from the reflector. The second spacer region is non-absorbingto the target wavelength, where at least a portion of the second spacerregion comprises a second electrical polarity opposite of the firstelectrical polarity. The reflector has a metal composition comprisingelemental aluminum. The resonant optical cavity light emitting devicehas an optical cavity between the reflector and a first surface of thesubstrate, the optical cavity comprising the first spacer region, thesecond spacer region and the light emitting region, where the opticalcavity has a total thickness less than or equal to K·λ/n. K is aconstant greater than zero ranging from about 0.25 to 10, such as K=0.5,1, 1.5, 2, etc. Lambda (λ) is the desired emission wavelength, and n isthe effective refractive index of the optical cavity at the desired(target) wavelength.

In some embodiments, K is a non-integer, such as a value 0<K<1, forinstance K ranging from 0.25 to less than 1. Such embodiments may havethe first spacer directly coupled to (i.e., directly on) the substrate,where there is no reflector (e.g., no DBR) between the first spacer andthe substrate. The thickness of the light emitting region has a finitethickness and is formed to exhibit an advantageous optical response thatis substantially absorbing and emissive to the operating wavelength. Forexample, the active region may comprise a quantum confining structurefor both electrons and holes (such as a superlattice or quantum well)which further introduces a finite penetration depth and refractive indexdue to excitonic effects. Therefore, without including the additionalcontribution of the active region optical response, exact integer valuesof K forming the cavity between the high reflector and partial reflectormay be undesirable. Furthermore, for the case of DBR structures used forreflective portions of the device there also exists a finite penetrationdepth of the operating wavelength into the interior of the DBR. As theDBR is an interferometric process, a finite number of periods and orlayers comprising the DBR are required to form the reflection and istherefore not defined as reflecting from the first layer of the DBR.Clearly, the parameter K can have non-integer values as described in thepresent embodiments.

In some embodiments, the separation distance of light emitting regionfrom the reflector is 1/10 to ½ of the total cavity thickness L_(CAV)and is typically limited to a smallest value by the thickness of thep-type region used for the p-up device structure. Alternatively, in someembodiments, the separation distance of the light emitting region fromthe reflector is ½ to 9/10 and is typically limited to a largest valueby the thickness of the p-type region used for the p-down devicestructure.

In some embodiments, the substrate is optically transparent to thetarget wavelength. In some embodiments, the substrate is opaque to thetarget wavelength, the substrate having an optical aperture through thesubstrate. In some embodiments, at least one of the light emittingregion, the first spacer region, and the second spacer region eachcomprise a superlattice. In some embodiments, the lateral dimensions ofthe device perpendicular to the plane of the layers comprising thedevice are in the range of less than or equal to 5 microns, and greateror equal to 0.1 microns.

FIGS. 13-15 demonstrate the use of the present methods for modelingfurther device geometries. For example, in FIGS. 13A-13B a side wallangle that tapers toward the substrate is modeled, having an angle of−15° in this embodiment. In FIGS. 14A-14B a side wall angle that tapersaway the substrate is modeled, having an angle of +45°. That is, for theangled side walls of FIGS. 14A-14B, a cross-sectional area of the firstspacer region is greater than a cross-sectional area of the secondspacer region. These configurations will impact the emissioncharacteristics of the device and can be optimized through thesimulations.

The devices of FIGS. 13 and 14 have approximately square cross-sectionsin a horizontal plane, as seen by the fact that the widths of thevertical cross-section in FIG. 13A is approximately the same as FIG.13B, and similarly for FIGS. 14A and 14B. In contrast, FIGS. 15A-15Bshow a rectangular geometry, where the width of the device in thex-dimension (FIG. 15A) is greater than that in the z-direction (FIG.15B). The LER position L_(SEP) is half of the cavity length. FIG. 15Cshows the emission output of the device of FIGS. 15A-15B, showing that apeak of the magnitude of radiation emission approximately coincides withthe desired wavelength XEM. Therefore, this device geometry is afeasible design choice for the LED.

In certain embodiments, it is desirable that the LED shape isasymmetrical (that is, non-axisymmetric) rather than symmetrical, suchas to produce optical emission polarization that is linearly polarizedor elliptically polarized. This serves to polarize the radiation emittedby the device and tends to enhance efficiency of the device. That is,rectangular shapes enable a preferred polarized radiation to be producedin a preferred direction. FIG. 17 shows a device 1700 that utilizesasymmetric geometries for selective polarization. The device 1700includes a plurality of rectangular or fin-like LED structures 1705parallel to each other on a substrate 1710, where LED structures 1705may be similar to, for example, the structures of FIGS. 15A-15B. The topsurfaces of the LED structures 1705—which are planes parallel to thesubstrate 1710—are non-axisymmetric. Breaking the axial symmetry andconfining the light in the plane of the layers creates a preferredpolarization field to be vertically emitted (arrows 1720) from thedevices. Placing the plurality of structures displaced horizontally amultiple of half wavelengths apart further reinforces the constructiveinterference of the emitted radiation field. The arrows 1725 and 1730represent the in-plane optical polarizations, with the relativemagnitude representing the degree of selectivity in a particulardirection due to the geometrical shape of the device fin 1705. That is,an asymmetric fin geometry 1705 would produce an elliptically polarizedTE-like emission pattern directed in a direction 1720.

In other embodiments, a symmetric geometry may be chosen to create mixedpolarization as shown in FIGS. 18A-18C, as an alternative to using anasymmetric structure like that shown in FIG. 17 to achieve selectivepolarization. If the device is symmetric such as having a horizontalcross-section being cylindrical, hexagonal or annular in shape, thenradiation emitted is randomly polarized whereas rectangular-shaped(asymmetric) devices result in the emission of polarized orientedradiation in a particular direction. Thus, the emission output may beconstrained as an ideal single mode emission output at an observationplane. This may be achieved by a cylindrical device having a diameterof, for example, approximately 2 μm for a III-N SL material system asshown in FIG. 18A, which provides plan and cross-sectional diagrams ofcylindrically symmetric example optical cavity devices having differentdiameters D_1<D_2<D_3. The vertical layers include an output coupler1805 and an optical emission region 1810 positioned at a distance A fromthe high reflector 1815. The plan views show the axisymmetric circularcross-sections in a plane parallel to the output coupler substrate 1805.For a given A, and operating wavelengths that are not well matched tothe diameter, the near field spatial emission pattern is multimode 1820and 1825. For the case of the diameter D_1 being well matched to theoperating wavelength, the spatial emission profile is single mode orquasi—single mode 1830. As in the previous embodiments, the cavity ofthe device is formed between an aluminum backside reflector (1815) and atransparent substrate (1805). Devices can be constructed by etching orlateral oxidation if using III-N into III-oxides of lower refractiveindex. That is, lateral optical confinement can be created by selectivespatial conversion of III-N by oxidation process creating anadvantageously low refractive index region and insulative region. Thelow refractive index region or etched region can be used to furtherlimit the planar extent of the 3D structure and thus further modify theallowed number of modes. Rotationally symmetric (i.e., disc) resonatorsproduce transmission modes of mixed E_(X) and E_(Z) polarization,including unpolarized and circularly polarized. The allowed opticalmodes within the volume of the device are therefore controlled toproduce optimal coupling to vertical propagating modes. FDTD simulationsas described above may be used to optimize the device dimensions, suchas to choose a desired disc radius for a given cavity length. Forexample, FIG. 18B illustrates a cylindrical vertical resonant opticalcavity LED having a radius R_(CAV) of 0.57 μm. A desirable singleoptical mode output is seen in at the exit plane, as demonstrated by thepeak emission being centered in the disk and gradually decreasingoutward. FIG. 18C simulates a radius R_(CAV) of 0.85 μm, where anundesirable multi-mode output is seen due to the concentric rings ofvarying intensity.

In other embodiments, a Fabry Perot interferometer effect may be used toincrease emission efficiency by placing a partially reflective mirror(e.g., 5%) above the substrate and between the optically transparentsubstrate and the first spacer region. That partial mirror and thereflective Al allow radiation to interfere positively to enhance theradiation emitted by the device. This is shown in FIG. 19 for the simplecavity structure 1905, where a partially reflective mirror 1920 acts asan output coupling material for the dielectric cavity 1925 and highreflectance mirror 1930. The example optical cavity 1905 is formed usingmetallic mirrors and the dielectric semiconductor cavity 1925 thatcomprises aluminum nitride. The normal incidence reflection spectrum1950 is for a semitransparent 13 nm aluminum with AlN dielectric cavityand highly reflecting 100 nm aluminum reflector that forms the verticaloptical cavity 1905, for the case of L_cav=m*λ/(2*n) and a desiredoperating wavelength of λ=240 nm (as indicated by optical mode 1955),where m=odd integer and n is the refractive index of AlN. The thicknessof the semitransparent aluminum output coupling mirror 1920 is optimizedfor a thickness of 13 nm to achieve a sharp optical mode 1955. Thespectral dependence of normal incidence 1910 and reflected light 1915 isshown in the graph 1950 for different cavity thicknesses. For example,the optical modes of the device 1905 as a function of the thickness ofcavity 1925 are determined in the reflectance spectrum 1950 for thecases of (m=1) 1935, (m=3) 1940 and (m=5) 1945, as shown in FIG. 19. Theoptical mode 1955 can therefore be utilized advantageously for outcoupling of optical energy generated from within the cavity.

The flowchart 2000 of FIG. 20 describes embodiments of methods formanufacturing a resonant optical cavity light emitting device of thevarious configurations and using the simulation methods describedherein. In step 2010, a substrate is provided. In step 2020, a firstspacer region is provided, the first spacer being coupled to thesubstrate. The first spacer region is non-absorbing to a target emissiondeep ultraviolet wavelength, where at least a portion of the firstspacer region comprises a first electrical polarity. A light emittingregion on the first spacer region is provided in step 2030, the lightemitting region being configured to emit the target emission deepultraviolet wavelength. A second spacer region is provided on the lightemitting region in step 2040, the second spacer region beingnon-absorbing to the target emission deep ultraviolet wavelength. Atleast a portion of the second spacer region comprises a secondelectrical polarity opposite of the first electrical polarity. In step2050 a reflector is provided, the reflector being coupled to the secondspacer region. The reflector has a metal composition comprisingelemental aluminum. The light emitting region is positioned at aseparation distance from the reflector. In step 2060 the resonantoptical cavity light emitting device is simulated using athree-dimensional electromagnetic spatial and temporal simulator. Thesimulation is based on the light emitting region being at the separationdistance from the reflector. In step 2070 it is determined if anemission output at an exit plane relative to the substrate meets apredetermined criterion. The predetermined criterion is selected from i)an optical extraction efficiency at the exit plane with respect to thelight generated from the light emission region at the target wavelength,ii) a target spatial profile of light emitted at the exit plane at thetarget wavelength, and iii) light extracted from the device beingemitted substantially perpendicular to the substrate.

In some embodiments of the methods of flowchart 2000, thethree-dimensional electromagnetic spatial and temporal simulator is afinite-difference time-domain simulator. In some embodiments, theresonant optical cavity light emitting device may comprise an electricalstructure of a p-i-n diode and is a vertical cavity surface lightemitting diode (VCSLED). In some embodiments, at least one of the lightemitting region, the first spacer region, and the second spacer regionmay each comprise a superlattice. In some embodiments, the resonantoptical cavity light emitting device may be configured as a p-up device,the first electrical polarity being n-type and the second electricalpolarity being p-type, where the optical reflector comprises purealuminum, and where a compound semiconductor and metallic contact isprovided on the second spacer region. In some embodiments, the resonantoptical cavity light emitting device may be configured as a p-downdevice, the first electrical polarity being p-type and the secondelectrical polarity being n-type, and where the optical reflector is ametal electrical contact for the resonant optical cavity light emittingdevice.

In some embodiments of the methods of flowchart 2000, the substrate maybe optically transparent, where at the target wavelength, a firstrefractive index of the optically transparent substrate is less than orequal to a second refractive index of the first spacer region. In someembodiments, a cross-section of the resonant optical cavity lightemitting device in a plane parallel to the substrate isnon-axisymmetric, providing an optical emission polarization that islinearly polarized or elliptically polarized. In some embodiments, across-section of the resonant optical cavity light emitting device in aplane parallel to the substrate is axisymmetric, providing opticalemission polarization that is unpolarized or circularly polarized.

In some embodiments of the methods of flowchart 2000, the resonantoptical cavity light emitting device has an optical cavity between thereflector and a first surface of the substrate, the optical cavitycomprising the first spacer region, the second spacer region and thelight emitting region; where the substrate is optically transparent, andwhere the optical cavity has an total thickness comprising the distancefrom the first surface of the substrate to the reflector, the totalthickness being less than or equal to K·λ/n. The total thickness of theoptical cavity may be less than or equal to the desired emissionwavelength, thereby forming a sub-wavelength resonator. The method mayfurther comprise selecting the target wavelength, selecting a materialfor the optically transparent substrate, selecting a thickness of thefirst spacer region, and selecting a thickness of the second spacerregion to match the target wavelength to at least one allowed opticalmode formed by the optical cavity and the reflector.

In various embodiments, the three-dimensional ROCLED may be constructedby a process including laterally patterning the layered structure toform a 3D device by using at least one of (i) spatially selectivesubtractive etching of a portion of the layered structure; (ii)spatially selective high energy implantation of foreign atomic speciesinto the layered structure; (iii) spatially selective transformation ofthe materials comprising the layered structure by transforming theinitial material into another form of material composition that isdissimilar to the initial material; and (iv) spatially selective coatingthe exposed sidewalls of the 3D structure using at least one lowrefractive index and electrically insulating material, wherein the lowrefractive index material at the operating deep ultraviolet emissionwavelength is less than the mean refractive index of the layerstructure. The patterning process is performed such that electronic andoptical confinement and or isolation of the device is achieved. In someembodiments, in-situ deposition on a second spacer region of an opticalreflector may be performed prior to patterning and forming the 3Ddevice, forming at least a portion of a final reflector structure duringthe layer formation process of the layered structure. In someembodiments, the 3D device construction process may include depositionof a high reflector after the layer formation step, and then patterningthe 3D device. In some embodiments, the device is manufactured by usingan in-situ reflectance spectrum monitoring of the layered structureduring film formation to determine the optical cavity thickness, andtuning of the optical cavity thickness to a predetermined value bycontrolling the thickness of the second spacer region prior todeposition of at least a portion of the optical reflector.

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, thesubstrate is transparent to at least the operating deep ultravioletemission wavelength. The transparent substrate may have a refractiveindex at the operating deep ultraviolet emission wavelength that is lessthan the average refractive index of the layered structure materials atthe operating deep ultraviolet emission wavelength. The transparentsubstrate may also be patterned to form an optical element selected froman optical concentrator (such as a lens), diffractive element (such as,a planar diffraction grating element or planar Bessel beam concentricaperture). In other embodiments, the substrate is opaque and ispatterned to form an optical aperture beneath at least a portion of thesaid layered structure and the high reflector. The opaque substrate maybe patterned to form an optical aperture that substantially removes allof the opaque substrate within the aperture, where formed opticalaperture may extend through the substrate and terminate to at least theinterface between the substrate and the first spacer region. A secondoptical reflector may be formed within the formed optical aperture thatis formed by depositing a reflector structure. The device may comprisean optical cavity formed by sandwiching a layered structure between anupper and a lower reflector, wherein one of the reflectors is of highreflectance at the operating deep ultraviolet emission wavelength, andthe other reflector is partially reflecting at the operating deepultraviolet emission wavelength.

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, thereflector material comprises metallic aluminum, providing highreflectivity and low loss at the operating deep ultraviolet emissionwavelength. In some embodiments, the reflector material is selected froma periodic quarter wavelength bi-layered stack comprising opticalmaterials selected from substantially transparent compositions at theoperating deep ultraviolet emission wavelength and having refractiveindex difference of at least 0.1 between the bi-layer pairs. In someembodiments, a reflector is a metallic-dielectric metamaterial where thereflector metamaterial is selected from a periodic quarter wavelengthstack comprising materials selected from substantially transparentmaterial compositions and metallic aluminum composition; where one ofthe quarter wavelength stack compositions is selected from at least oneof aluminum nitride (AlN), magnesium fluoride (MgF₂),aluminum-oxy-nitride (AlO_(x)N_(y)), aluminum oxide Al₂O₃, and calciumfluoride (CaF₂); and another quarter wavelength stack composition isselected from a multilayered stack comprising an optically transparentmaterial and metallic aluminum (Al), with the optically transparentmaterial selected from at least one of aluminum nitride (AlN), magnesiumfluoride (MgF₂), aluminum-oxy-nitride (AlO_(x)N_(y)), aluminum oxideAl₂O₃, and calcium fluoride (CaF₂). In some embodiments, the reflectoris a distributed dielectric Bragg reflector selected from a periodicquarter wavelength stack comprising at least two dissimilarsubstantially transparent material compositions selected from at leasttwo of aluminum nitride (AlN), magnesium fluoride (MgF₂),aluminum-oxy-nitride (AlO_(x)N_(y)), aluminum oxide (Al₂O₃), and calciumfluoride (CaF₂).

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, an opticalcavity is formed between the high reflector and the substrate, where theoptical cavity comprises the first spacer region, light emitting regionand the second spacer region. The optical cavity may have a thicknessthat is less than the operating deep ultraviolet emission wavelength,thereby producing a sub-wavelength cavity. The optical cavity may have athickness that is comparable to the operating deep ultraviolet emissionwavelength, thereby producing a nano-cavity. The optical may have athickness that is greater than the operating deep ultraviolet emissionwavelength, thereby producing a sub-micron cavity.

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, the devicehas lateral dimensions being less than 2 microns thereby forming anoptical lateral confinement structure. In certain embodiments, at leastone of the two lateral dimensions is less than 2 microns thereby formingan optical lateral confinement structure. In certain embodiments, thedevice has at least one of the two lateral dimensions being less than 2microns thereby forming an optical lateral confinement structure andhaving an output spatial optical mode that is substantially single mode.In certain embodiments, the device has at least one of the two lateraldimensions being less than 2 microns thereby forming an optical lateralconfinement structure and having an output spatial optical mode that issubstantially a Bessel mode.

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, the deviceis a P-up structure having a p-type conductivity for at least a portionof first spacer region structure is disposed near the substrate; and an-type conductivity for at least a portion of the second spacer regionstructure is disposed near the reflector. The light emitting region issubstantially intrinsic conductivity or not intentionally doped and ispositioned at allowed optical modes of the cavity. The light emittingregion is preferentially positioned near the p-type conductivity regionrelative to the n-type conductivity region due to the low mobility ofholes compared to electrons.

In some embodiments of constructing the three-dimensional ROCLED by aprocess including laterally patterning the layered structure, the deviceis a P-down structure having a n-type conductivity for at least aportion of first spacer region structure is disposed near the substrate;and a p-type conductivity for at least a portion of the second spacerregion structure is disposed near the reflector. The light emittingregion is substantially intrinsic conductivity or not intentionallydoped and is positioned at allowed optical modes of the cavity. Thelight emitting region is preferentially positioned near the p-typeconductivity region relative to the n-type conductivity region due tothe low mobility of holes compared to electrons.

Further embodiments for creating optical cavities for deep UV operationare shown in FIGS. 24-39. FIG. 24 shows an example optical cavity formedusing metallic mirrors and dielectric semiconductor cavity comprisingaluminum nitride. Normal incidence reflectance from ametal-dielectric-metal optical cavity wherein the front metal Isemitransparent aluminum of thickness 15 nm and the rear high reflectoris thick aluminum of 100 nm. The curves show the effect of varying thedielectric cavity thickness L_cav=m*λ/(2*n), where the operatingwavelength λ is selected as 240 nm and the refractive index n isselected for Aluminum Nitride (AlN) at 240 nm, and m=1, . . . , 7. Thereflectance minima 2405, 2410, 2415, 2420, 2425, 2430 are used to tunethe emission wavelength of the device when the dielectric cavitycontains an optical emissive region. For deep ultraviolet wavelengthsoperating between 200 nm to 300 nm, the reflectors are preferablyselected from Aluminum metal thereby reducing the penetration depth oflight.

FIG. 25 is an example optical cavity formed using metallic mirrors anddielectric semiconductor cavity comprising aluminum nitride. The normalincidence reflection spectrum 2500 is for a semitransparent aluminum/AlNdielectric cavity/highly reflecting aluminum reflector forming avertical optical cavity, for L_cav=m*λ/(2*n), where m=3 and λ=240 nm.The thickness of the semitransparent aluminum output coupling mirror isoptimized as indicated by curve 2510 for a thickness of 13 nm.Disadvantageous detuning of the optical resonance of the cavity occursfor 2 nm variation 2505 (L=15 nm) and 2515 (L=11 nm) from the optimalthickness of 13 nm.

FIG. 26 is an example optical cavity 2600 using the device of FIG. 16A,where the spacer layers 1602 and 1604 have a total length of 800 nm(i.e. the sum of the spacer layers 1602 and 1604 is 800 nm), the workfunction metal 1606 is platinum and has a length of 2 nm, aluminumreflector 1607 has a length of 600 nm, and GaN test layer EHR 1603 is0.5 nm thick. Arrow 2645 indicates an incident white spectrum on thesapphire substrate, which results in a reflected spectrum 2650. Thereflectance spectrum of the optical device of FIG. 26 is shown in FIG.27. The effect of the thickness of the p-type GaN layer 1605 (L_pGaN) asa function of wavelength is shown in the curves 2715 and 2710 of FIG. 27for the case of L_pGaN=0 nm and 25 nm, respectively. The pGaN absorberreduces the coupling of Al to the cavity. The profound absorption of theparasitic absorption of the pGaN layers alters the optical cavity asshown. High absorption of pGaN effectively reduces the influence of therear aluminum high reflector and thus inhibits cavity operation.

In FIG. 28, the vertical optical modes within the cavity of FIGS. 24 and25 are shown for three cases of p-type gallium nitride thickness. Theplots of 2805, 2810 and 2815 describe spatial-spectral absorption as afunction of position of the EHR region 1603 (FIG. 16A) within thedielectric cavity relative to the sapphire substrate surface. The threeplots 2805, 2810 and 2815 are for the same device structure but varyingpGaN thickness and fixed high work function layer thickness=2 nm. Theabsorption in the EHR region is seen to decrease as the parasiticabsorption of the pGaN layer increases due to increasing L(pGaN)=0, 5and 10 nm. That is, the pGaN absorption reduces the effect of the Alhigh reflector and reduces the optical mode strength within the cavity.Comparing regions 2830 and 2850 shows that the pGaN absorption not onlyreduces the absorption of the EHR when resonant with the cavity modesbut also redshifts the cavity modes slightly. Therefore, if the deviceis designed to emit at wavelength 265 nm (2820) for the case ofL(pGaN)=0, then increasing the L(pGaN)=10 nm will result in an apparentshift in emission wavelength >26 5 nm. Clearly, for a given EHR position1630, the resonant modes of high absorption at specific wavelengths forlight incident from the transparent sapphire substrate are observed. Theintroduction of a thin absorbing region of GaN is shown to modify thestrength and wavelength position of the resonance. Therefore, the EHR ispreferentially selected to be positioned at positions of high absorptionfor a given operating design wavelength. For example, the optical modes2830 in configuration map 2805 are advantageous positions for the EHRwithin the cavity to operate at wavelength 2820.

FIG. 29 is an example reflector structure for an optical device 2905comprising an Aluminum Nitride dielectric cavity. The p-type contactstack comprises AlN-based dielectric cavity 2925, a p-typedielectric/semiconductor 2920 of thickness L_pGaN, an ohmic metal layer2915 that is partially absorbing at the operating wavelength, and analuminum high reflector 2910. For example, the thickness of AlN-basedcavity is L_ALN=800 nm. The configuration of FIG. 29 may alternativelybe an AlGaN, GaN, or SL(AlN/GaN) cavity instead of AlN.

FIGS. 30A and 30B show the effect of the optical reflectance at a designwavelength of 240 nm as observed through the sapphire substrate of thedevice in FIG. 26. FIG. 30A shows the reflectance 3005 as a function ofpGaN thickness (L_pGaN) for the cases of various titanium ohmic metalthickness. The ideal case 3010 is shown and the least preferred case is3015. FIG. 30B shows the reflectance 3020 vs. titanium ohmic metalthickness (L_Ti), with the best case curve 3025 and least preferred3030. The calculation shows the optimization process for deviceoperation for a given p-type GaN and ohmic metal configuration.Therefore, the necessary use of advantageously ohmic electrical contactmaterials, such as 2915, 2920 of FIG. 29 but having disadvantageouslyoptical absorption property can be optimized by minimizing the thicknessof the materials between the highly reflecting Al mirror 2910 and thedielectric cavity and preserve the optical cavity effect of the presentdisclosure.

FIG. 31 is a schematic representation of optical cavities that arepossible using dielectric mirrors (device 3105) ormetal-dielectric-metal (device 3150). The dielectric mirrors of devicetype 3105 comprise quarter wavelength thicknesses of at least twodissimilar materials 3110 and 3115 of sufficient number of periods tocreate the desired reflectance at an operating wavelength. Thedielectric cavity 3120 of thickness 3130 and the EHR 3125 are shown. Theemission spectrum of the EHR is matched to the optical modes of thecavity (example as shown in FIG. 28). Device 3150 ismetal-dielectric-metal optical cavity including the high reflector metal3155 and dielectric cavity 3160. Dielectric cavity 3160 includes an EHR3125 that also acts as the light emission region and may also provide anemission gain spectrum as shown in graph 3140. The partiallyreflecting/transmitting thin film metal output coupler 3165 completesthe optical cavity device 3150. Graph 3140 represents the longitudinaloptical modes created by the mirrors and cavity material, whereas thegain spectrum is the emission profile of EHR 3125. Overlap of the gainspectrum with an allowed optical mode enables light generated fromwithin the cavity to escape the devices as on optical beam 3145.

FIG. 32 shows a reflectance spectrum 3200 for example dielectricreflectors 3230 comprising quarter wavelength bilayers of aluminumnitride (AlN) 3235 and aluminum oxide (Al₂O₃) 3240. The reflectancespectrum 3200 within a desired wavelength band 3220 is shown for variousdielectric mirrors of 21-periods 3205, 9-periods 3210 and 6-periods3215. These dielectric mirrors can be used to form optical cavitydevices of the type 3105 of FIG. 31. The penetration depth 3170 ofwavelengths (FIG. 31) is much deeper than an equivalent metal-dielectricdevice. Region 3225 represents an out of band portion of the spectrumthat does not produce advantageous reflectance increase. The region ofwavelength band 3220 represents the in-band reflectance spectrum that isdesired for increased reflectance.

FIG. 33 shows the reflectance spectrum of an exampledielectric-semiconductor mirror that can be used to form an opticalcavity of the present disclosure. The optical cavity comprises AlN, andthe high reflector comprises a 30-period Al₂O₃/AlN quarter wavelengthstack and a 10.5-period output coupler for a design wavelength. In thereflectance spectrum 3300, the optical cavity mode 3305 is shown at 250nm.

FIG. 34 shows the reflectance spectrum of another exampledielectric-semiconductor mirror that can be used to form an opticalcavity of the present disclosure. The optical cavity comprises AlN, andthe high reflector comprises a 20-period MgF₂/AlN quarter wavelengthstack and a 5.5-period output coupler for a design wavelength. In thereflectance curve 3400, the optical cavity mode 3405 is shown at 250 nm.The large refractive index difference between magnesium fluoride (MgF₂)and aluminum nitride (AlN) provides superior optical cavity mode 3405compared to 3305.

FIG. 35 is an example vertically emitting optical cavity device 3520comprising a substrate 3530 and optical cavity 3525. Optical cavity 3525includes Al₂O₃/AlN mirrors, an AlN cavity with Al₂O₃ apertures tocontrol the transverse optical mode and an etched region 3550 in theoutput coupling mirror. The device structure 3520 is simulated with anEHR position within the middle of the AlN cavity, and the FDTD resultsare shown in simulation map 3500 and electric field graph 3540. Theoptical emission beam 3503 shows the out coupled wavelength. The on-axiselectric field intensity 3505 is plotted in graph 3540 showing thepenetration depth into the dielectric mirrors. The refractive indexvariation 3545 shows the high and low refractive indices used for theoptical cavity.

FIGS. 36A-36B show yet a further example of a deep ultraviolet mirrorformed using a metalo-semiconductor combination 3620 of thinnanostructured aluminum metal layers and dielectric layers. Each thinaluminum metal layer is sandwiched between wide bandgap energy aluminumnitride semiconductor/dielectric. The effective refractive index n_effof the bi-layered slab is designed by selecting the thicknesses of thealuminum layer 3630 (L_Al) and the AlN layer 3635 (L_AlN), as shown inFIG. 36B. For operating wavelength of λ=250 nm as shown in graph 3600 ofFIG. 36A, the bilayer pairs comprise the unit cell of the periodicmetalo-dielectric slab (L_Al, L_AlN)=(0.8 nm, 1 nm) 3610 and (1.6 5 nm,2 nm) 3615 resulting in n_eff=1.4. Selecting the total number of periodsfor a given (L_Al, L_AlN) pair to equal an optical thickness ofλ/(4*n_eff) enables utility as a reflective material operating in thedeep UV range. Furthermore, using the metalo-dielectric slab functioningas a quarter wavelength layer further comprising a distributed Braggreflector (DBR) can be used advantageously to create a highly reflectingmedium suitable for an optical cavity. That is, the metalo-dielectricperiodic thin film layers comprise a superlattice SL[Al/AlN] forming onetype of effective index material of the DBR. An example of a DBRsuitable for optical cavity herein is formed comprising at least twodifferent two materials types M1 and M2, wherein M1 and M2 are bothquarter wavelength thickness materials of refractive indices n_M1 andn_M2, where ΔM=M2−M2, and further comprising a number P of periodicrepetition stack of [M1/M2]_1, [M1/M2]_2, . . . , [M1/M2]_P and the M1comprises the disclosed SL[Al/AlN] slab which is of total thicknessequal to one quarter wavelength and M2 is selected from a quarterwavelength thick layer of bulk AlN. An advantage of such ametalo-dielectric DBR is the large change in refractive index Δn betweenM1 and M2 and thus a small number of periods P is required to achievehigh reflectivity in the deep UV range. This DBR type is particularlyadvantageous during in-situ deposition of AlN-based optoelectronicdevices, such as LEDs and vertically emitting lasers of the type 3105(refer FIG. 31). The metalo-dielectric DBR superlattice (e.g., asuperlattice of Al and AlN layers) corresponding to the graph and deviceof FIGS. 36A-36B can be used as either a high reflector or partialreflector in the embodiments described throughout this disclosure.

FIGS. 37A-37B show an example optical cavity 3700 operating in the deepUV range of the type 3150 (FIG. 31). The highly reflecting aluminum rearreflector 3705 sandwiches an AlN-based dielectric cavity 3715 by a lowrefractive index transparent substrate 3720. The effective outputcoupler is the formed by the step index between the refractive indicesof the dielectric 3715 and substrate 3720. An optional buriedsemitransparent reflector can be positioned between the dielectric andthe substrate. A nanostructured emissive/absorption region 3710 ispositioned a distance from the high reflector 3705 within the AlN-baseddielectric cavity. The region 3710 can be a quantum well or superlatticematerial comprising aluminum gallium nitride (AlGaN) designed to achievean optical emission/absorption spectrum as shown in FIG. 37B. Forexample, a layered structure of materials AlN/GaN/AlN forming a quantumwell or superlattice SL[AlN/GaN] can be used for region 3710. Thespectral width of the predetermined spectrum 3730 and 3735 and the peakposition with respect to wavelength are controlled by selecting thematerial thickness and composition in the angstrom to nanometer regimeforming quantum confined states, such as excitons. The insertion of thenonlinear optical region 3710 acts as the EHR region. The EHR spectruminfluences the optical cavity formed by the dielectric 3715 betweensubstrate 3720 and reflector 3705.

In some embodiments, the thickness L_(SEP) of the second spacer regioncan be less than 100 nm; that is, the light emitting region can bepositioned at a separation distance from the reflector from0<L_(SEP)≤100 nm. For a second spacer region having a high refractiveindex material and operating at a deep UV wavelength (e.g., 200-280 nm),these aspects will limit the possible thicknesses of the second spacerregion. For example, for a second spacer region of AlN, the refractiveindex n of AlN is n=2.4 at a target wavelength of λ=250 nm.Consequently, the high refractive index of the second spacer region cansupport a larger number of optical wavelength periods therebyeffectively making the optical length larger than for example a halfcycle of operating wavelength. Conversely, in some embodiments thesecond spacer region includes a p-type or hole source layer. In general,holes have lower mobility than electrons and the transport length ofholes can be extremely short in defective crystalline material or largebandgap compositions. To efficiently inject holes into the active regionit is advantageous to position the hole source layer relatively close tothe active layer. The structure of the second spacer layer therefore canbe constructed with a small separation distance (e.g., 0<L_(SEP)≤100 nm)so that the optical properties of the cavity and the electronictransport property of the materials are optimized.

FIGS. 38 and 39 show the operation of examples devices with0<L_(SEP)≤100 nm. In FIG. 38, the operation of the optical cavitydescribed in FIG. 37A is shown by the reflectance map 3800 as viewedthrough the transparent sapphire substrate 3720. The position 3805 ofthe EHR region 3710 dramatically influences the operation of the device.For a given operating wavelength, for example, 240 nm or 250 nm, themaxima and minima can be selected advantageously by positioning the EHRwithin the AlN-based optical cavity, as shown. For example, the incidentwhite spectrum indicated by arrow 2645 on the sapphire substrate resultsin a reflected spectrum 2650 (FIG. 26) as shown in the reflectance map3800. The reflectance as viewed through the sapphire is influenced bythe optical cavity effect as described herein. A high absorption withinthe EHR region 3710 is achieved by positioning the region 3710 at anoptical cavity mode selected by 3805 and results in a reflectanceminimum, for example 3810 and 3815. The reciprocal process is alsounderstood wherein the EHR is an optically emissive region and the goalis to extract maximum light out through the sapphire substrate.Therefore, by designing the optical cavity and placing the EHR region3710 advantageously at an absorption maximum will result in a highextraction efficiency optical emitter using the optical cavity.

FIG. 39 is an absorption map 3900 within only the EHR (i.e., LER) region3710 of the device of FIG. 37A, where the EHR region 3710 is scannedvertically through the AlN-based dielectric cavity at a position 3905relative to the dielectric-substrate interface. Clearly, for anoperating wavelength of 250 nm, the absorption maxima, for example, 3910and 3915, are advantageous positions for the EHR region at approximately25 nm and 75 nm in this embodiment. Other positions of the EHR regionwithin the AlN-based dielectric cavity will result in poor opticalextraction efficiency of light generated from the EHR region at 250 nm.

In further embodiments, resonant optical cavity LEDs may have an opaquesubstrate without an optical aperture, where light is emitted out of theopposite (top) end of the stack rather than through the substrate. FIG.40 is a vertical cross-sectional view of one such embodiment, where adevice 4000 has an opaque substrate 4010, a first reflective layer 4015on the substrate 4010, a first spacer region 4020 a/b coupled to thefirst reflective layer 4015, a light emitting region 4030 on the firstspacer region 4020 a/b, a second spacer region 4040 a/b on the LER 4030and a second reflective layer 4050 coupled to the top of the devicestack. As indicated by the arrows in FIG. 40, light 4060 emitted fromthe light emitting region 4030 is output through the second reflectivelayer 4050, either directly or after being reflected by first reflectivelayer 4015 and/or second reflective layer 4050.

First reflective layer 4015 is in practice challenging to construct, aslayer 4015 must possess three properties simultaneously. Firstly, layer4015 must exhibit high optical reflectance at the operating emissionwavelengths, with reflectance in the range of 70% to 100%, andpreferably approaching 90% to 100%. Secondly, if the opaque substrate4010 is electrically conductive and the device 4000 is a verticallyconductive device then layer 4015 must exhibit sufficient electricalconductivity to enable electrical transport between adjacent n-type orp-type conductivity layer 4020 a and the underlying conductive andopaque substrate 4010. Thirdly, layer 4015 must exhibit crystalstructure commensurate with both that of the substrate 4010 and activelayers forming the device 4000. In preference all three properties aresought to be obtained by the materials selected for layer 4015. For thecase where layer 4015 comprises a plurality of dissimilar opticalrefractive index materials—forming, for example, a DBR stack—then allthree requirements are sought.

Conventionally, non-absorbing materials are utilized for a DBR which arewider bandgap materials compared to those used within the active regionof the device. Wider bandgap materials typically result indisadvantageously lower electrical conductivity compared to lowerbandgap materials. This results in resistive losses across layer 4015for the case of a fully vertical conductive device utilizing an opaqueconductive substrate. Materials selection to achieve high opticaltransparency (low optical loss at the operating wavelength) andsufficient change in refractive index are further constraints to form aDBR. For example, devices operating in the deep UV range limits thematerial selection. The materials used for the DBR may be similar or ofa different crystal type to those of the active region and/or substrate.For the case of a DBR using the same crystal type as the active regionand/or substrate, the change in available refractive index is small andthus necessitates the use of many repeating periods to construct asuitable DBR section reflectance. This further increases the thicknessof layer 4015 and may also increase the resistive losses. For the casewhere crystal type layers are used for layer 4015 that are dissimilarfrom the substrate or active layer crystal type, a further challengeoccurs in the ability to form low crystal defect density depositedlayers. Crystal defects limit the electrical and optical properties ofthe materials and subsequent materials deposited upon them.

Therefore, the inclusion of first reflective layer 4015 in the presentdevices has new challenges compared to conventional devices. In someembodiments, manufacturing methods such as wafer bonding are utilized toovercome these manufacturing issues. For example, wafer bonding can beutilized for the case where the opaque substrate provides good thermalconductivity and electrical conductance. Yet another example utilizesthe material properties of opaque substrate 4010 to create therefractive index change between the layer 4015 and the substrate 4010,thus providing improved reflectance. Yet a further example is the use ofan epitaxially deposited DBR structure to form layer 4015 on thecrystalline and opaque substrate 4010. Subsequent deposition of theactive layers or wafer bonding of the active region starting at 4020 ato the surface of layer 4015 may be performed.

Opaque substrate 4010 may be, for example, a semiconductor type, suchas, Silicon, Germanium, Silicon-Carbide, Gallium-Oxide,Aluminium-Gallium-Oxide, Aluminium-Oxide, Gallium-Nitride,Lithium-Niobate, and GaAs. For example, a p-type GaN (p-GaN) substratemay be utilized for a closer crystal structure match of deposited layersforming the active layers and/or the reflective layer 4010. This enablesa vertically conductive device but offers an opaque substrate, such asfor a device operating at wavelengths where the p-type GaN substrate isoptically absorbing. Other substrate materials such as pure metal can beused such as single crystal Aluminum and the like. First reflectivelayer 4015 is a DBR that is highly reflective to the target wavelength,such as having a reflectivity of greater than 90% for the targetwavelength produced by the active region (LER 4030). The firstreflective layer 4015 may comprise, for example, Al, MgF₂ or AlN, andmay exclude AlGaN materials in some embodiments. A metal-dielectric DBRis also possible, where the use of two dissimilar materials (e.g., “A”and “B”) are used for creating two different refractive index layers(e.g., “n_A” and “n_B”), and further used to construct the period stackof the DBR (that is A-B-A-B- . . . stack). Material A may, for example,be a quarter wavelength of AlN, and material B may comprise asuperlattice material itself comprising a periodic stack of ultra-thinAluminium metal and a wide bandgap material (e.g. AlN). The thickness ofmaterial B is also selected to be of quarter wave thickness. The DBRformed in this manner enables use of direct epitaxial film formationmethod and provides the ability to form the layer 4015 in-situ to thedevice formation process.

First spacer region 4020 a/b is non-absorbing to the target wavelength,and at least a portion of the first spacer region 4020 a/b comprises afirst electrical polarity. In some embodiments, the first spacer region4020 a/b may include two layers, a doped layer 4020 a of the firstelectrical polarity (e.g., n or p conductivity) and an undoped layer4020 b. Similarly, second spacer region 4040 a/b is non-absorbing to thetarget wavelength, and at least a portion of the second spacer region4040 a/b comprises a second electrical polarity opposite of the firstelectrical polarity. In some embodiments, the second spacer region 4040a/b may include a doped layer 4040 a of the second electrical polarity(e.g., p or n conductivity) and an undoped layer 4040 b. Secondreflective layer 4050 is coupled to the second spacer region 4040 a/b.The second reflective layer 4050 is a thin film metal reflector, such asless than 15 nm thick, of a material that is highly reflective for thetarget wavelength. For example, for deep UV wavelengths, the secondreflective layer 4050 may be a metal composition comprising elementalaluminum.

The device 4000 may have resonant optical cavity characteristics asdescribed throughout this disclosure. For example, an optical cavitybetween the second reflective layer 4050 and first reflective layer 4015of the resonant optical cavity light emitting device 4000 may includethe first spacer region 4020 a/b, the second spacer region 4040 a/b andthe light emitting region 4030. The light emitting region 4030 ispositioned at a separation distance from the second reflective layer4050. The optical cavity has a total thickness less than or equal toK·λ/n. K is a constant ranging from 0.25 to 10, where λ is the targetwavelength and n is an effective refractive index of the optical cavityat the target wavelength.

In some embodiments of the device 4000 where the device is configured asa p-up device, the separation distance is 1/10 to ½ of the totalthickness of the optical cavity, the first electrical polarity isn-type, and the second electrical polarity is p-type. In someembodiments of the device 4000 where the device is configured as ap-down device, the separation distance is ½ to 9/10 of the totalthickness of the optical cavity, the first electrical polarity isp-type, and the second electrical polarity is n-type. In someembodiments, the separation distance is less than 100 nm. In someembodiments, at least one of the light emitting region 4030, the firstspacer region 4020 a/b, and the second spacer region 4040 a/b comprise asuperlattice.

FIG. 41 shows another embodiment of a resonant optical cavity LED havingan opaque substrate and emitting light out of a top end of the devicestack rather than through the substrate. In this verticalcross-sectional view, a device 4100 has an opaque substrate 4110, afirst reflective layer 4115 on the substrate 4110, a first spacer region4120 a/b coupled to the first reflective layer 4115, a light emittingregion 4130 on the first spacer region 4120 a/b, a second spacer region4140 a/b on the LER 4130, and a second reflective layer 4150 coupled tothe top of the device stack. As indicated by the arrows in FIG. 41,light 4160 emitted from the light emitting region 4130 is output throughthe second reflective layer 4150, either directly or after beingreflected by first reflective layer 4115 and/or second reflective layer4150. The same issues arise as described in relation to FIG. 40;however, reflective layer 4115 of FIG. 41 can be selected from a puremetallic reflector or a low optical refractive index material. Both ofthese criteria potentially limit the materials that can be selected forthe device owing to at least one of: a difference in crystal structuremismatch between substrate and/or active layers, poor electricalconductivity, and poor thermal conductivity. For example, in oneembodiment of the device 4100, epitaxial Gallium Oxide and AluminiumOxide are deposited on a single crystal Silicon substrate. The Siliconsubstrate is preferably of high electrical conduction but opticallyopaque. The layer 4115 may be selected from reflective metal ordielectric semiconductor and deposited as a quarter wavelength opticallyreflective layer for light emanating toward it and generated from theactive region. Such a device offers the ability for direct epitaxy of anAlGaN based active layer.

Substrate 4110 may be, for example, Silicon, Silicon-Carbide,Gallium-Oxide, Aluminium-Gallium Oxide, Gallium Arsenide, Germanium andother oxide substrates. The thickness of the substrate can be optionallythinned to enable electrical contact penetrations from the opaquesubstrate into the active region layers punctuating insulatingreflective layer 4115. First reflective layer 4115 is a thick metal filmhaving a metal composition comprising elemental aluminum and having athickness greater than 15 nm. First spacer region 4120 a/b isnon-absorbing to the target wavelength, and at least a portion of thefirst spacer region 4120 a/b comprises a first electrical polarity. Insome embodiments, the first spacer region 4120 a/b may include twolayers, a doped layer 4120 a of the first electrical polarity (e.g., nor p conductivity) and an undoped layer 4120 b. Similarly, second spacerregion 4140 a/b is non-absorbing to the target wavelength, and at leasta portion of the second spacer region 4140 a/b comprises a secondelectrical polarity opposite of the first electrical polarity. In someembodiments, the second spacer region 4140 a/b may include a doped layer4140 a of the second electrical polarity (e.g., p or n conductivity) andan undoped layer 4140 b. Second reflective layer 4150 is coupled to thesecond spacer region 4140 a/b. The second reflective layer 4150 is athin film metal reflector, such as less than 15 nm thick, of a materialthat is highly reflective for the target wavelength. For example, fordeep UV wavelengths, the second reflective layer 4150 may be a metalcomposition comprising elemental aluminum.

The device 4100 may have resonant optical cavity characteristics asdescribed throughout this disclosure. For example, an optical cavitybetween the second reflective layer 4150 and the first reflective layer4115 of the resonant optical cavity light emitting device 4100 mayinclude the first spacer region 4120 a/b, the second spacer region 4140a/b and the light emitting region 4130. The light emitting region 4130is positioned at a separation distance from the second reflective layer4150. The optical cavity has a total thickness less than or equal toK·λ/n. K is a constant ranging from 0.25 to 10, where λ is the targetwavelength and n is an effective refractive index of the optical cavityat the target wavelength.

In some embodiments of the device 4100 where the device is configured asa p-up device, the separation distance is 1/10 to ½ of the totalthickness of the optical cavity, the first electrical polarity isn-type, and the second electrical polarity is p-type. In someembodiments of the device 4100 where the device is configured as ap-down device, the separation distance is ½ to 9/10 of the totalthickness of the optical cavity, the first electrical polarity isp-type, and the second electrical polarity is n-type. In someembodiments, the separation distance is less than 100 nm. In someembodiments, at least one of the light emitting region 4130, the firstspacer region 4120 a/b, and the second spacer region 4140 a/b comprise asuperlattice.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those of ordinary skill in theart, without departing from the scope of the present invention, which ismore particularly set forth in the appended claims. Furthermore, thoseof ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention.

What is claimed is:
 1. A resonant optical cavity light emitting devicecomprising: a substrate; a first reflective layer on the substrate, thefirst reflective layer having a metal composition comprising elementalaluminum and having a thickness greater than 15 nm; a first spacerregion coupled to the first reflective layer, the first spacer regionbeing non-absorbing to a target emission deep ultraviolet wavelength(target wavelength), wherein at least a portion of the first spacerregion comprises a first electrical polarity; a light emitting region onthe first spacer region, the light emitting region being configured toemit the target wavelength; a second spacer region on the light emittingregion, the second spacer region being non-absorbing to the targetwavelength, wherein at least a portion of the second spacer regioncomprises a second electrical polarity opposite of the first electricalpolarity; and a second reflective layer coupled to the second spacerregion, the second reflective layer having a metal compositioncomprising elemental aluminum and having a thickness less than or equalto 15 nm; wherein the substrate is optically opaque to the targetwavelength; wherein the first spacer region, the light emitting regionand the second spacer region comprise AlN or AlGaN; wherein the lightemitting region is positioned at a separation distance from the secondreflective layer; and wherein the resonant optical cavity light emittingdevice has an optical cavity between the second reflective layer and thefirst reflective layer, the optical cavity comprising the first spacerregion, the second spacer region and the light emitting region, whereinthe optical cavity has a total thickness less than or equal to K·λ/n,wherein K is a constant ranging from 0.25 to 10, λ is the targetwavelength, and n is an effective refractive index of the optical cavityat the target wavelength.
 2. The device of claim 1, wherein theseparation distance is 1/10 to ½ of the total thickness of the opticalcavity; and wherein the resonant optical cavity light emitting device isconfigured as a p-up device, the first electrical polarity being n-type,the second electrical polarity being p-type.
 3. The device of claim 1,wherein the separation distance is ½ to 9/10 of the total thickness ofthe optical cavity; and wherein the resonant optical cavity lightemitting device is configured as a p-down device, the first electricalpolarity being p-type, the second electrical polarity being n-type. 4.The device of claim 1, wherein the first spacer region comprises AlN,the light emitting region comprises AlN or AlGaN, and the second spacerregion comprises AlN.
 5. The device of claim 1, wherein at least one ofthe light emitting region, the first spacer region, and the secondspacer region comprise a superlattice.
 6. The device of claim 1, whereinthe substrate is optically transparent to the target wavelength.
 7. Aresonant optical cavity light emitting device comprising: a substratethat is opaque to a target emission deep ultraviolet wavelength (targetwavelength); a first reflective layer on the substrate, the firstreflective layer being a distributed Bragg reflector (DBR) superlatticethat has a reflectivity of greater than 90% for the target wavelength; afirst spacer region coupled to the first reflective layer, the firstspacer region being non-absorbing to the target wavelength, wherein atleast a portion of the first spacer region comprises a first electricalpolarity; a light emitting region on the first spacer region, the lightemitting region being configured to emit the target wavelength; a secondspacer region on the light emitting region, the second spacer regionbeing non-absorbing to the target wavelength, wherein at least a portionof the second spacer region comprises a second electrical polarityopposite of the first electrical polarity; and a second reflective layercoupled to the second spacer region, the second reflective layer havinga metal composition comprising elemental aluminum and having a thicknessless than 15 nm; wherein the light emitting region is positioned at aseparation distance from the second reflective layer; and wherein theresonant optical cavity light emitting device has an optical cavitybetween the second reflective layer and the first reflective layer, theoptical cavity comprising the first spacer region, the second spacerregion and the light emitting region, wherein the optical cavity has atotal thickness less than or equal to K·λ/n, wherein K is a constantranging from 0.25 to 10, λ is the target wavelength, and n is aneffective refractive index of the optical cavity at the targetwavelength.
 8. The device of claim 7, wherein the DBR superlattice is ametal-dielectric stack.
 9. The device of claim 8, wherein a firstmaterial of the metal-dielectric stack and a second material of themetal-dielectric stack each have a quarter wavelength thickness.
 10. Thedevice of claim 8, wherein a first material of the metal-dielectricstack is AlN and a second material of the metal-dielectric stackcomprises aluminum metal.
 11. The device of claim 8, wherein a firstmaterial of the metal-dielectric stack is AlN and a second material is aperiodic stack of aluminum metal and AlN.
 12. The device of claim 7,wherein the substrate is a semiconductor material.
 13. The device ofclaim 7, wherein the substrate is a metal.
 14. The device of claim 7,wherein at least one of the light emitting region, the first spacerregion, and the second spacer region comprise a superlattice.
 15. Aresonant optical cavity light emitting device comprising: a substratethat is opaque to a target emission deep ultraviolet wavelength (targetwavelength); a first reflective layer on the substrate, the firstreflective layer comprising a dielectric semiconductor; a first spacerregion coupled to the first reflective layer, the first spacer regionbeing non-absorbing to the target wavelength, wherein at least a portionof the first spacer region comprises a first electrical polarity; alight emitting region on the first spacer region, the light emittingregion being configured to emit the target wavelength; a second spacerregion on the light emitting region, the second spacer region beingnon-absorbing to the target wavelength, wherein at least a portion ofthe second spacer region comprises a second electrical polarity oppositeof the first electrical polarity; and a second reflective layer coupledto the second spacer region, the second reflective layer having a metalcomposition comprising elemental aluminum and having a thickness lessthan 15 nm; wherein the light emitting region is positioned at aseparation distance from the second reflective layer; and wherein theresonant optical cavity light emitting device has an optical cavitybetween the second reflective layer and the first reflective layer, theoptical cavity comprising the first spacer region, the second spacerregion and the light emitting region, wherein the optical cavity has atotal thickness less than or equal to K·λ/n, wherein K is a constantranging from 0.25 to 10, λ is the target wavelength, and n is aneffective refractive index of the optical cavity at the targetwavelength.
 16. The device of claim 15, wherein the dielectricsemiconductor comprises gallium oxide and aluminum oxide.
 17. The deviceof claim 16, wherein the substrate is single crystal silicon.